Set http_proxy to retrieve the signed Release file
Set http_proxy to retrieve the signed Release file

This commit adds http_proxy=$http_proxy to use apt-get update through a
http-proxy.

Change-Id: Iee589f6d11b84991512493d22f0e3dbfe60736ec
Closes-Bug: #1412314

zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,takahashinobuyuki/diskimage-builder,rdo-management/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder