Use fallback URL for deploying tools for custom-built .zip

NexusKeywords.Deploy_Artifact was assuming that ${BUNDLE_URL}
points to a full Nexus installation.
But if the bundle under test comes from another source
(most notably patch test job on Jenkins, either RelEng or Sandbox)
there are no test tools on similar URLs.

In that case, fall back to default ODL Nexus (snapshot repository).

Change-Id: Icdd09da8ca219d26b42ca60e6aee0520220e9c31
Signed-off-by: Vratko Polak <vrpolak@cisco.com>