Ensure epel7 is installed only on rhel7/centos7
Ensure epel7 is installed only on rhel7/centos7

Change-Id: I5812812f7d2cb5ba2e310e33d6e01cda8823c1c4

takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,NaohiroTamura/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder,takahashinobuyuki/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,rdo-management/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,eantyshev/diskimage-builder,JioCloud/diskimage-builder,zioproto/diskimage-builder,varunarya10/diskimage-builder,JioCloud/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,NaohiroTamura/diskimage-builder,openstack/diskimage-builder