Check existence of rhel rpm key
Check existence of rhel rpm key

The key 'RPM-GPG-KEY-redhat-release' may not exist in image
e.g. when you use non-default image (for example, if you build CentOS
image using rhel element).

Fixes bug #1252697

Change-Id: Iae642abb7a7f76c5528750dafedd4fd4bda56fe2

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