Use DIB_EPEL_MIRROR when finding the epel-release package
Use DIB_EPEL_MIRROR when finding the epel-release package

download.fedoraproject.org uses dns round robin and occasionally
hits a bad server. Using DIB_EPEL_MIRROR when finding the
epel-release package will allow us to avoid it e.g. in ci.

Change-Id: I756223b3e669532476663c05e79c238449b8a0db

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