virt/image-install: merge changes from commit e564ff12

Merge the patch fixing the RHEL version checking in /distribution/virt/install task
into image-install task.

Change-Id: I8dc0fe33e8f74e6588462792ccadb40c346b0fc6
virt/image-install: merge changes from commit a28fdc3a

This is for applying the same patch as we have done to /distribution/virt/install,
to use NetworkManager on RHEL7 to set up network bridging.

Change-Id: Ib13b55be0b87ee991019f449e69653c5e49643df