Add 'builder-sandbox' job to Releng

Add 'builder-sandbox' job to Releng

Any changes slated for the 'sandbox' branch of this repo will be
published to https://sandbox.opnfv.org, with the exception of the
releng-ci jobs themselves. This will provide visibility to changes
slated for Jenkins and allow for easier understanding of how JJB works
for new users.

- Step [2/2] in publishing jobs to the Jenkins sandbox

JIRA: RELENG-91

Change-Id: I94c28081b1d96e5eae76a4f1f11dcf5cb9b6cf35
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>