Update to newer Centos7 images as announced at [1].
Changes included:
* update to GA image
* respect TMP_DIR for image working directory
* use suffix-match rather than prefix-match to build RAW_IMAGE name
to be more robust against weird names
* partition layout work-arounds not needed any-more, all one
partition.
Resulting image booted locally with kvm and virtualbox (after
conversion to vdi)
[1] https://www.redhat.com/archives/rdo-list/2014-August/msg00045.html
Closes-Bug: #1363146
Change-Id: Ie24033468b78587ea87188ee1b843b26895798ff