Initial centos7 support for build-image.sh

Add some filtering to 55-cache-devsatck-repos to handle centos7, which
actually gets setup with a DISTRO_NAME of rhel7 in devstack

Update the readme file to highlight the options for building other
images.

"DISTRO=centos7 ./tools/build-image.sh" creates a workable centos7
qcow image with this change and a d-i-b that includes
Ie24033468b78587ea87188ee1b843b26895798ff

Change-Id: Ida38cba0a303ab77432d5d354f1952e00ebfa00e
Merge "Initial centos7 support for build-image.sh"