ceph-source-dist: Support for ceph-release-pipeline

ceph-source-dist: Support for ceph-release-pipeline

Again, the changes needed here are:
 - For a Release Build, we are building a tarball based off a
   version commit that has been created by the ceph-tag job
   then pushed to ceph-releases.git.  So instead of taking
   the BRANCH from an upstream Jenkins job, we're reading
   the version commit SHA1 from the sha1.txt file that ceph-tag
   wrote then checking /that/ out and building the tarball from it.
   So some var writing and reading was shuffled around.
 - We do not want this job to support checking out from ceph-private.git
   or ceph-releases.git directly.  Instead, if ceph-release-pipeline
   passed RELEASE_BUILD=true, only then can you clone from those private
   repos.  So we override CEPH_REPO and set chacra_url to the chacra
   instance that doesn't automatically prune repos (chacra.ceph.com).

Signed-off-by: David Galloway <david.galloway@ibm.com>