Due to changes in centos7 element to use package-installs the order
of yum-repos in pre-install.d needs to be changed, to make yum repos
available before package-installs is called to install packages.
Change-Id: I47b584506465d90247f30bad515a32323e52180e
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
