mirror/ubuntu: use HTTP proxy for downloading the packages
mirror/ubuntu: use HTTP proxy for downloading the packages

Pass the http_proxy/HTTP_PROXY environment variables (if set) to
the temporary chroot where the packages get actually downloaded.

blueprint support-ubuntu-trusty

Change-Id: Idbd86e0edab3531d098f7fef0dd0c4abe1798e81

ddepaoli3/fuel-main-dev,AnselZhangGit/fuel-main,SergK/fuel-main,dancn/fuel-main-dev,huntxu/fuel-main,SergK/fuel-main,ddepaoli3/fuel-main-dev,Fiware/ops.Fuel-main-dev,dancn/fuel-main-dev,zhaochao/fuel-main,SmartInfrastructures/fuel-main-dev,ddepaoli3/fuel-main-dev,SmartInfrastructures/fuel-main-dev,huntxu/fuel-main,eayunstack/fuel-main,SmartInfrastructures/fuel-main-dev,AnselZhangGit/fuel-main,AnselZhangGit/fuel-main,eayunstack/fuel-main,zhaochao/fuel-main,teselkin/fuel-main,zhaochao/fuel-main,teselkin/fuel-main,ddepaoli3/fuel-main-dev,eayunstack/fuel-main,zhaochao/fuel-main,huntxu/fuel-main,stackforge/fuel-main,stackforge/fuel-main,Fiware/ops.Fuel-main-dev,Fiware/ops.Fuel-main-dev,SmartInfrastructures/fuel-main-dev,dancn/fuel-main-dev,stackforge/fuel-main,SergK/fuel-main,dancn/fuel-main-dev,teselkin/fuel-main,teselkin/fuel-main,zhaochao/fuel-main,Fiware/ops.Fuel-main-dev,AnselZhangGit/fuel-main