Vagrantfile: Fix using Vagrant with LXC >= 2.
Vagrantfile: Fix using Vagrant with LXC >= 2.

Apparently LXC 2 removed support for the `-B best` option in
lxc-create, and Vagrant hasn't been updated appropriately yet, so we
need to add a workaround to explicitly specify a backing store.

Fixes #718.

This manifested as errors of the form:

"""
There was an error executing ["sudo",
"/usr/local/bin/vagrant-lxc-wrapper", "lxc-create", "-B", "best",
"--template",
"/home/tabbott/.vagrant.d/gems/gems/vagrant-lxc-1.2.1/scripts/lxc-template",
"--name", "zulip_default_1461801696512_85064", "--", "--tarball",
"/home/tabbott/.vagrant.d/boxes/fgrehm-VAGRANTSLASH-trusty64-lxc/1.2.0/lxc/rootfs.tar.gz",
"--config",
"/home/tabbott/.vagrant.d/boxes/fgrehm-VAGRANTSLASH-trusty64-lxc/1.2.0/lxc/lxc-config"]
"""

susansls/zulip,JPJPJPOPOP/zulip,dattatreya303/zulip,Juanvulcano/zulip,cosmicAsymmetry/zulip,JPJPJPOPOP/zulip,isht3/zulip,amyliu345/zulip,aakash-cr7/zulip,vabs22/zulip,blaze225/zulip,peguin40/zulip,Jianchun1/zulip,paxapy/zulip,amyliu345/zulip,j831/zulip,peiwei/zulip,umkay/zulip,brainwane/zulip,mohsenSy/zulip,umkay/zulip,Juanvulcano/zulip,Galexrt/zulip,Juanvulcano/zulip,rht/zulip,mohsenSy/zulip,amanharitsh123/zulip,arpith/zulip,Diptanshu8/zulip,punchagan/zulip,Frouk/zulip,jrowan/zulip,niftynei/zulip,KingxBanana/zulip,punchagan/zulip,christi3k/zulip,Galexrt/zulip,sonali0901/zulip,timabbott/zulip,eeshangarg/zulip,sup95/zulip,dattatreya303/zulip,krtkmj/zulip,brainwane/zulip,ahmadassaf/zulip,grave-w-grave/zulip,christi3k/zulip,andersk/zulip,reyha/zulip,timabbott/zulip,synicalsyntax/zulip,Diptanshu8/zulip,verma-varsha/zulip,SmartPeople/zulip,niftynei/zulip,mahim97/zulip,Vallher/zulip,krtkmj/zulip,synicalsyntax/zulip,vaidap/zulip,tommyip/zulip,zulip/zulip,peiwei/zulip,Jianchun1/zulip,kou/zulip,vikas-parashar/zulip,Diptanshu8/zulip,calvinleenyc/zulip,isht3/zulip,brockwhittaker/zulip,Frouk/zulip,eeshangarg/zulip,kou/zulip,ryanbackman/zulip,isht3/zulip,jrowan/zulip,kou/zulip,hackerkid/zulip,sharmaeklavya2/zulip,mahim97/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,aakash-cr7/zulip,krtkmj/zulip,showell/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brockwhittaker/zulip,AZtheAsian/zulip,peiwei/zulip,Frouk/zulip,dhcrzf/zulip,reyha/zulip,sup95/zulip,Galexrt/zulip,j831/zulip,vabs22/zulip,sharmaeklavya2/zulip,niftynei/zulip,christi3k/zulip,dattatreya303/zulip,brockwhittaker/zulip,dattatreya303/zulip,niftynei/zulip,Diptanshu8/zulip,sup95/zulip,arpith/zulip,krtkmj/zulip,christi3k/zulip,JPJPJPOPOP/zulip,Galexrt/zulip,paxapy/zulip,punchagan/zulip,dhcrzf/zulip,brockwhittaker/zulip,hackerkid/zulip,hackerkid/zulip,Diptanshu8/zulip,shubhamdhama/zulip,zacps/zulip,PhilSk/zulip,JPJPJPOPOP/zulip,Jianchun1/zulip,samatdav/zulip,vabs22/zulip,vaidap/zulip,umkay/zulip,amanharitsh123/zulip,Frouk/zulip,souravbadami/zulip,kou/zulip,ahmadassaf/zulip,vikas-parashar/zulip,sharmaeklavya2/zulip,Vallher/zulip,punchagan/zulip,PhilSk/zulip,PhilSk/zulip,jphilipsen05/zulip,vabs22/zulip,sonali0901/zulip,zulip/zulip,vaidap/zulip,susansls/zulip,dawran6/zulip,brockwhittaker/zulip,rht/zulip,mohsenSy/zulip,jphilipsen05/zulip,TigorC/zulip,reyha/zulip,eeshangarg/zulip,jrowan/zulip,jackrzhang/zulip,ryanbackman/zulip,verma-varsha/zulip,sharmaeklavya2/zulip,zulip/zulip,tommyip/zulip,grave-w-grave/zulip,amanharitsh123/zulip,Vallher/zulip,arpith/zulip,hackerkid/zulip,dawran6/zulip,rishig/zulip,isht3/zulip,SmartPeople/zulip,umkay/zulip,reyha/zulip,joyhchen/zulip,joyhchen/zulip,tommyip/zulip,TigorC/zulip,umkay/zulip,zacps/zulip,brainwane/zulip,arpith/zulip,joyhchen/zulip,timabbott/zulip,blaze225/zulip,samatdav/zulip,cosmicAsymmetry/zulip,mohsenSy/zulip,sup95/zulip,Frouk/zulip,TigorC/zulip,peiwei/zulip,souravbadami/zulip,peguin40/zulip,showell/zulip,showell/zulip,ahmadassaf/zulip,rht/zulip,shubhamdhama/zulip,aakash-cr7/zulip,mahim97/zulip,umkay/zulip,samatdav/zulip,blaze225/zulip,souravbadami/zulip,Frouk/zulip,umkay/zulip,shubhamdhama/zulip,hackerkid/zulip,christi3k/zulip,synicalsyntax/zulip,dhcrzf/zulip,ryanbackman/zulip,brainwane/zulip,vabs22/zulip,synicalsyntax/zulip,zulip/zulip,jackrzhang/zulip,isht3/zulip,j831/zulip,brockwhittaker/zulip,verma-varsha/zulip,verma-varsha/zulip,Diptanshu8/zulip,jrowan/zulip,verma-varsha/zulip,brainwane/zulip,dhcrzf/zulip,dattatreya303/zulip,andersk/zulip,verma-varsha/zulip,jrowan/zulip,cosmicAsymmetry/zulip,SmartPeople/zulip,peiwei/zulip,jphilipsen05/zulip,TigorC/zulip,samatdav/zulip,dhcrzf/zulip,tommyip/zulip,tommyip/zulip,samatdav/zulip,susansls/zulip,j831/zulip,AZtheAsian/zulip,rht/zulip,amyliu345/zulip,SmartPeople/zulip,punchagan/zulip,KingxBanana/zulip,vikas-parashar/zulip,ahmadassaf/zulip,brainwane/zulip,rht/zulip,rishig/zulip,Galexrt/zulip,AZtheAsian/zulip,synicalsyntax/zulip,joyhchen/zulip,arpith/zulip,jackrzhang/zulip,arpith/zulip,zulip/zulip,andersk/zulip,ahmadassaf/zulip,Galexrt/zulip,jackrzhang/zulip,sonali0901/zulip,cosmicAsymmetry/zulip,Vallher/zulip,rishig/zulip,mahim97/zulip,ryanbackman/zulip,zulip/zulip,vikas-parashar/zulip,dawran6/zulip,peguin40/zulip,zacps/zulip,mohsenSy/zulip,krtkmj/zulip,AZtheAsian/zulip,KingxBanana/zulip,Frouk/zulip,dawran6/zulip,Jianchun1/zulip,zacps/zulip,jphilipsen05/zulip,hackerkid/zulip,rht/zulip,zacps/zulip,andersk/zulip,mahim97/zulip,reyha/zulip,amanharitsh123/zulip,souravbadami/zulip,TigorC/zulip,ahmadassaf/zulip,calvinleenyc/zulip,Jianchun1/zulip,peguin40/zulip,synicalsyntax/zulip,grave-w-grave/zulip,jainayush975/zulip,Juanvulcano/zulip,zulip/zulip,calvinleenyc/zulip,ahmadassaf/zulip,synicalsyntax/zulip,AZtheAsian/zulip,jphilipsen05/zulip,tommyip/zulip,blaze225/zulip,showell/zulip,AZtheAsian/zulip,j831/zulip,joyhchen/zulip,KingxBanana/zulip,mohsenSy/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,Juanvulcano/zulip,souravbadami/zulip,andersk/zulip,shubhamdhama/zulip,jainayush975/zulip,grave-w-grave/zulip,jainayush975/zulip,isht3/zulip,dawran6/zulip,jrowan/zulip,rht/zulip,paxapy/zulip,PhilSk/zulip,jainayush975/zulip,paxapy/zulip,jackrzhang/zulip,hackerkid/zulip,vikas-parashar/zulip,timabbott/zulip,krtkmj/zulip,showell/zulip,paxapy/zulip,blaze225/zulip,christi3k/zulip,jainayush975/zulip,vaidap/zulip,amyliu345/zulip,jphilipsen05/zulip,blaze225/zulip,Vallher/zulip,Vallher/zulip,amyliu345/zulip,JPJPJPOPOP/zulip,cosmicAsymmetry/zulip,kou/zulip,grave-w-grave/zulip,vikas-parashar/zulip,punchagan/zulip,Galexrt/zulip,peguin40/zulip,susansls/zulip,dawran6/zulip,krtkmj/zulip,KingxBanana/zulip,sup95/zulip,peiwei/zulip,zacps/zulip,PhilSk/zulip,vabs22/zulip,rishig/zulip,showell/zulip,susansls/zulip,dhcrzf/zulip,sharmaeklavya2/zulip,Jianchun1/zulip,joyhchen/zulip,andersk/zulip,j831/zulip,rishig/zulip,sup95/zulip,peguin40/zulip,kou/zulip,reyha/zulip,shubhamdhama/zulip,jackrzhang/zulip,cosmicAsymmetry/zulip,sonali0901/zulip,ryanbackman/zulip,rishig/zulip,SmartPeople/zulip,timabbott/zulip,eeshangarg/zulip,sonali0901/zulip,susansls/zulip,calvinleenyc/zulip,paxapy/zulip,jackrzhang/zulip,timabbott/zulip,niftynei/zulip,aakash-cr7/zulip,SmartPeople/zulip,sonali0901/zulip,amanharitsh123/zulip,niftynei/zulip,amyliu345/zulip,vaidap/zulip,jainayush975/zulip,calvinleenyc/zulip,brainwane/zulip,TigorC/zulip,aakash-cr7/zulip,dattatreya303/zulip,eeshangarg/zulip,PhilSk/zulip,Juanvulcano/zulip,mahim97/zulip,samatdav/zulip,amanharitsh123/zulip,grave-w-grave/zulip,peiwei/zulip,punchagan/zulip,showell/zulip,Vallher/zulip,ryanbackman/zulip,KingxBanana/zulip,souravbadami/zulip,JPJPJPOPOP/zulip,dhcrzf/zulip,sharmaeklavya2/zulip,vaidap/zulip,calvinleenyc/zulip,aakash-cr7/zulip