Initial centos7 support

Initial support for a centos7 image.

This is separate to rhel7 because the major differences are things
like repo and image locations, which are always going to be different.
We should merge any real changes into the redhat-common layers.

Apart from the added support files in centos7/*, the other change is
mostly modifications to redhat-common's extract-image to handle
different partition layouts of the centos7 image.

Change-Id: I943abe5ff0a803f36eda266a79af0d9220edcae7