The default value was set in the centos7 element, but not
exported, which caused issues in rpm-distro. Also changed
a test in rpm-distro to only check for DIB_RELEASE > 22
if it's fedora.
Closes-Bug: #1477172
Change-Id: Ib6f4227411c2e8f1965c3b78bc318512c59a7876
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
