Add RPM_REPO_URL global variable

The ta-rpm-deploy.sh script is using ALT_NEXUS_URL to point to this
URL, which is causing issues elsewhere in some jobs. What we really
want is to set this globally, and have scripts use this global var
rather than setting their own.

Change-Id: I9b0017b42421610e4537063140b3d6d37421c21b
Signed-off-by: Eric Ball <eball@linuxfoundation.org>