Ensure plugin installs respect global mirror setting
Ensure plugin installs respect global mirror setting

Fixes #163

juliandunn/jenkins,cru-Luis-Rodriguez/jenkins,Dispader/jenkins,Sauraus/jenkins,dshvedchenko/jenkins,martinb3-cookbooks/jenkins,Dispader/jenkins,rchekaluk/jenkins,mlongob/jenkins,j-c-h-e-n-g/jenkins,bhalothia/jenkins,gshively11/jenkins,epu/epu-cb-jenkins,mknapik/jenkins,RobertNorthard/jenkins,mknapik/jenkins,DataDog/jenkins,autoclone/jenkins,mknapik/jenkins,mlongob/jenkins,umar85/jenkins,ebsco/jenkins,chef-cookbooks/jenkins,daften/cookbook-jenkins,chef-cookbooks/jenkins,cru-Luis-Rodriguez/jenkins,epu/epu-cb-jenkins,autoclone/jenkins-cookbook,juliandunn/jenkins,rexut/opscode-cookbooks-jenkins,StephenKing/chef-cookbook-jenkins,bhalothia/jenkins,dshvedchenko/jenkins,mlongob/jenkins,bhalothia/jenkins,opscode-cookbooks/jenkins,RobertNorthard/jenkins,b-dean/jenkins,umar85/jenkins,developerinlondon/jenkins,autoclone/iptables,samdunne/jenkins,StephenKing/chef-cookbook-jenkins,daften/cookbook-jenkins,b-dean/jenkins,smedefind/jenkins,autoclone/jenkins,Tiger66639/jenkins,daften/cookbook-jenkins,gshively11/jenkins,b-dean/jenkins,opscode-cookbooks/jenkins,opscode-cookbooks/jenkins,autoclone/jenkins-cookbook,martinb3-cookbooks/jenkins,Sauraus/jenkins,developerinlondon/jenkins,rexut/opscode-cookbooks-jenkins,ebsco/jenkins,developerinlondon/jenkins,rchekaluk/jenkins,autoclone/iptables,Tiger66639/jenkins,epu/epu-cb-jenkins,umar85/jenkins,Dispader/jenkins,rchekaluk/jenkins,rexut/opscode-cookbooks-jenkins,cru-Luis-Rodriguez/jenkins,Tiger66639/jenkins,dshvedchenko/jenkins,martinb3-cookbooks/jenkins,j-c-h-e-n-g/jenkins,Sauraus/jenkins,ebsco/jenkins,RobertNorthard/jenkins,samdunne/jenkins,StephenKing/chef-cookbook-jenkins,autoclone/iptables,Piousbox-cookbooks/jenkins,j-c-h-e-n-g/jenkins,autoclone/jenkins-cookbook,samdunne/jenkins,chef-cookbooks/jenkins,juliandunn/jenkins,autoclone/jenkins,gshively11/jenkins,Piousbox-cookbooks/jenkins,Piousbox-cookbooks/jenkins