Updated docs with environment variables for virtualbox driver
Updated docs with environment variables for virtualbox driver

Signed-off-by: Tom Barlow <adf75c2c2ec965e68e8d4752c9742146f0b6495c@gmail.com>

steeve/machine,unodba/machine,JimGalasyn/docker.github.io,StackPointCloud/machine,dskora/machine,troy0820/docker.github.io,jhoos/machine,dnephin/machine,erikdejonge/machine,cab105/machine,pdxjohnny/machine,joelea/machine,alexisbellido/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,gdevillele/docker.github.io,troy0820/docker.github.io,thaJeztah/docker.github.io,rupakg/machine,pdxjohnny/machine,docker/machine,rwilliams/machine,dudymas/machine,robertbreker/machine,xiaohui/machine,ehazlett/machine,jgsqware/machine,13W/machine,titanlien/machine,feelobot/machine,OpenNebula/machine,atsaki/machine,yanzay/machine,jzwlqx/denverdino.github.io,wzymaster/machine,menglingwei/denverdino.github.io,13W/machine,sanscontext/docker.github.io,LuisBosquez/docker.github.io,joaofnfernandes/docker.github.io,ekristen/machine,sanscontext/docker.github.io,jlusiardi/machine,pottava/machine,allingeek/machine,dskora/machine,wlan0/machine,askb/machine,troy0820/docker.github.io,legal90/machine,hypriot/machine,shin-/docker.github.io,LaynePeng/machine,skatsuta/machine,aperepel/machine,cjellick/machine,zehicle/machine,amylindburg/machine,exoscale/docker-machine,whilp/machine,blueyed/machine,aduermael/docker.github.io,joeuo/docker.github.io,phiroict/docker,OpenNebula/machine,atsaki/machine,flavio/machine,shin-/docker.github.io,rowancarr/machine,evq/machine,brandoncc/machine,denverdino/machine,LuisBosquez/docker.github.io,bfirsh/machine,bgokden/machine,docker/docker.github.io,denverdino/docker.github.io,cab105/machine,joaofnfernandes/docker.github.io,exoscale/docker-machine,kacole2/machine,ibuildthecloud/machine,denverdino/docker.github.io,rawkode/machine,atsaki/machine-2,tpires/machine,bdwill/docker.github.io,databus23/machine,sanscontext/docker.github.io,paultag/machine,yut148/machine,pragkent/machine,flavio/machine,anshulr/machine,londoncalling/machine,j3tm0t0/machine,tyrken/machine,BSWANG/denverdino.github.io,jeanlaurent/machine,phiroict/docker,jmelis/machine,docker-zh/docker.github.io,denverdino/docker.github.io,clintonskitson/machine,wlan0/machine,robvanmieghem/machine,wlan0/machine,rwilliams/machine,johnstep/docker.github.io,dmp42/machine,rupakg/machine,emsi/machine,jmelis/machine,StackPointCloud/machine,menglingwei/denverdino.github.io,denverdino/denverdino.github.io,denverdino/docker.github.io,dgageot/machine,larsla/machine,mjtrefethen/machine,BSWANG/denverdino.github.io,shot/machine,bdwill/docker.github.io,miqui/machine,johnstep/docker.github.io,MHBauer/machine,crunchywelch/machine,johnstep/docker.github.io,rowancarr/machine,OpenNebula/machine,larsla/machine,akolpakov/machine,miqui/machine,joaofnfernandes/docker.github.io,BSWANG/denverdino.github.io,dudymas/machine,s7v7nislands/machine,juliengarcia/machine,bookerzzz/machine,shot/machine,ppadala/machine,rillig/docker.github.io,euank/machine,cvstebut/machine,joelea/machine,crunchywelch/machine,duglin/machine,skippbox/kmachine,zchee/machine,Herbis/machine,abstractitptyltd/machine,JimGalasyn/docker.github.io,robertbreker/machine,nunofgs/docker-machine,carolynvs/machine,wzymaster/machine,asalhamed/machine,ksasi/machine,denverdino/machine-1,shubheksha/docker.github.io,imikushin/machine,shubheksha/docker.github.io,vovimayhem/machine,jlusiardi/machine,brandoncc/machine,rillig/docker.github.io,docker/docker.github.io,zchee/machine,gdevillele/docker.github.io,pottava/machine,ravolt/docker-machine,clintonskitson/machine,cpuguy83/machine,JohnKDay/machine,phiroict/docker,shot/machine,moxiegirl/machine,jarosser06/machine,gdevillele/docker.github.io,anweiss/docker.github.io,zehicle/machine,tpires/machine,docker/docker.github.io,slowteetoe/machine,euank/machine,jgsqware/machine,rawkode/machine,bookerzzz/machine,bgokden/machine,moxiegirl/machine,londoncalling/machine,legal90/machine,menglingwei/machine,cvstebut/machine,otherjason/machine,voutilad/machine,akolpakov/machine,joaofnfernandes/docker.github.io,dnephin/machine,JohnKDay/machine,schatt/machine,databus23/machine,xinxian0458/machine,BSWANG/denverdino.github.io,shin-/docker.github.io,mjtrefethen/machine,askb/machine,jpressnell/machine,jeanlaurent/machine,cgvarela/machine,atsaki/machine-2,beni55/machine,chantra/machine,jzwlqx/denverdino.github.io,TrumanLing/machine,anweiss/docker.github.io,shubheksha/docker.github.io,aduermael/docker.github.io,gerred/machine,JohnKDay/machine,hironobu-s/machine,alindeman/machine,sjfloat/machine,ibuildthecloud/machine,sausheong/machine,denverdino/denverdino.github.io,menglingwei/denverdino.github.io,kechol/machine,beni55/machine,errordeveloper/machine,laveolu/machine,aduermael/docker.github.io,Kast0rTr0y/docker-machine,docker-zh/docker.github.io,denverdino/denverdino.github.io,binario200/machine,jlusiardi/machine,docker-zh/docker.github.io,bwahn/machine,thaJeztah/docker.github.io,ksasi/machine,thaJeztah/machine,JimGalasyn/docker.github.io,docker/docker.github.io,shubheksha/docker.github.io,trecloux/machine,yanzay/machine,miqui/machine,justintung/machine,soleo/machine,denverdino/denverdino.github.io,danix800/docker.github.io,evq/machine,jsirois/machine,LuisBosquez/docker.github.io,tyrken/machine,jsirois/machine,joeuo/docker.github.io,TrumanLing/machine,trecloux/machine,menglingwei/machine,shubheksha/docker.github.io,marsmensch/machine,jeanlaurent/machine,carloreggiani/machine,jasisk/machine,yut148/machine,anweiss/docker.github.io,mark-adams/machine,londoncalling/docker.github.io,wzymaster/machine,bwahn/machine,BSWANG/denverdino.github.io,rwilliams/machine,bdwill/docker.github.io,schatt/machine,Herbis/machine,shin-/docker.github.io,ggiamarchi/machine,CenturyLinkLabs/machine,jzwlqx/denverdino.github.io,trecloux/machine,ubiquityhosting/machine,ddaze/machine,databus23/machine,davidstosik/machine,soleo/machine,vovimayhem/machine,NeilW/machine,cjellick/machine,shin-/docker.github.io,binario200/machine,menglingwei/denverdino.github.io,jkingyens/machine,danix800/docker.github.io,LaynePeng/machine,Parallels/docker-machine,j3tm0t0/machine,slowteetoe/machine,ksasi/machine,anweiss/docker.github.io,joeuo/docker.github.io,usmanismail/machine,Qwinix/docker-machine-vmware,robinbb/machine,dweomer/docker-machine,jkingyens/machine,wenlock/machine,slashk/machine,tyrken/machine,hironobu-s/machine,bdwill/docker.github.io,ocean/machine,atsaki/machine,denverdino/machine,StackPointCloud/machine,jhoos/machine,pottava/machine,soleo/machine,xinxian0458/machine,beni55/machine,ppadala/machine,bfirsh/machine,jzwlqx/denverdino.github.io,titanlien/machine,thaJeztah/docker.github.io,jeffellin/machine,pragkent/machine,carloreggiani/machine,anshulr/machine,dmp42/machine,joaofnfernandes/docker.github.io,cgvarela/machine,danix800/docker.github.io,deitch/machine,bwahn/machine,seegno-forks/machine,mjhea0/machine,robvanmieghem/machine,mjhea0/machine,ravolt/docker-machine,kacole2/machine,s7v7nislands/machine,aduermael/docker.github.io,paultag/machine,allingeek/machine,ocean/machine,denverdino/machine-1,sjfloat/machine,JimGalasyn/docker.github.io,titanlien/machine,londoncalling/docker.github.io,dgageot/machine,jsirois/machine,errordeveloper/machine,gdevillele/docker.github.io,sebadiaz/machine,jeffellin/machine,anweiss/docker.github.io,Qwinix/docker-machine-vmware,sebadiaz/machine,joeuo/docker.github.io,yanzay/machine,deitch/machine,marsmensch/machine,surgeforward/machine,kacole2/machine,robertbreker/machine,amylindburg/machine,denverdino/docker.github.io,LuisBosquez/docker.github.io,hypriot/machine,otherjason/machine,ggiamarchi/machine,wenlock/machine,jmelis/machine,rillig/docker.github.io,CenturyLinkLabs/machine,unodba/machine,aperepel/machine,hironobu-s/machine,juliengarcia/machine,carolynvs/machine,csakoda/machine,skippbox/kmachine,errordeveloper/machine,robinbb/machine,kunalkushwaha/machine,zchee/machine,jhoos/machine,johnstep/docker.github.io,jeffellin/machine,denverdino/machine,ddaze/machine,dweomer/docker-machine,Kast0rTr0y/docker-machine,Parallels/docker-machine,rowancarr/machine,imikushin/machine,voutilad/machine,robvanmieghem/machine,denmat/machine,thaJeztah/machine,NeilW/machine,alexisbellido/docker.github.io,alexisbellido/docker.github.io,surgeforward/machine,mark-adams/machine,frapposelli/machine,londoncalling/docker.github.io,skatsuta/machine,menglingwei/machine,johnstep/docker.github.io,sanscontext/docker.github.io,ubiquityhosting/machine,jzwlqx/denverdino.github.io,londoncalling/docker.github.io,rillig/docker.github.io,SvenDowideit/machine,dudymas/machine,rdgreis/machine,jkingyens/machine,JimGalasyn/docker.github.io,joyent/machine,ppadala/machine,faijaz/docker-machine,rdgreis/machine,usmanismail/machine,gdevillele/docker.github.io,emsi/machine,jasisk/machine,asalhamed/machine,jgsqware/machine,alindeman/machine,ubiquityhosting/machine,MHBauer/machine,laveolu/machine,cvstebut/machine,bookerzzz/machine,otherjason/machine,denverdino/denverdino.github.io,sanscontext/docker.github.io,ehazlett/machine,pdxjohnny/machine,xiaohui/machine,csakoda/machine,nunofgs/docker-machine,carolynvs/machine,chantra/machine,kunalkushwaha/machine,moxiegirl/machine,phiroict/docker,cpuguy83/machine,xinxian0458/machine,denmat/machine,skatsuta/machine,alexisbellido/docker.github.io,allingeek/machine,sjfloat/machine,vovimayhem/machine,juliengarcia/machine,bdwill/docker.github.io,danix800/docker.github.io,docker-zh/docker.github.io,bgokden/machine,slashk/machine,steeve/machine,askb/machine,kechol/machine,justintung/machine,gerred/machine,alexisbellido/docker.github.io,tpires/machine,wenlock/machine,phiroict/docker,menglingwei/denverdino.github.io,thaJeztah/machine,feelobot/machine,gerred/machine,pragkent/machine,docker-zh/docker.github.io,kunalkushwaha/machine,joeuo/docker.github.io,davidstosik/machine,xiaohui/machine,seegno-forks/machine,SvenDowideit/machine,cab105/machine,joyent/machine,thaJeztah/docker.github.io,faijaz/docker-machine,usmanismail/machine,londoncalling/docker.github.io,Grynn/machine,Grynn/machine,duglin/machine,jpressnell/machine,docker/machine,jarosser06/machine,frapposelli/machine,NeilW/machine,erikdejonge/machine,troy0820/docker.github.io,blueyed/machine,sausheong/machine,davidstosik/machine,LuisBosquez/docker.github.io,whilp/machine,faijaz/docker-machine,sebadiaz/machine,abstractitptyltd/machine,ekristen/machine,Grynn/machine