Update to newer Centos7 images

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
Merge "Update to newer Centos7 images"