Extend packagecloud-push for build-dir

Extend packagecloud-push for build-dir

Extend script packagecloud-push.sh with environment variable BUILD_DIR.
Populate BUILD_DIR from JJB config parameter build-dir, required for
cmake anyhow.  This limits the search to a single directory.

Add build-dir default to docker-ci template, "export-packages"

Revise export script to use a constant value for build-dir,
also "export-packages" in $WORKSPACE.

Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>
Change-Id: I3da170246fb5868e644756e80c20b07bf68f55f9