Add gate-magnetodb-devstack job

Add an initial devstack job for the MagnetoDB project.
It just installs magnetodb's devstack integration and runs devstack.
That just ensures that the devstack integration doesn't blow up just
trying to start up the service(s). There's not much else to test yet,
but this is a starting point to build on.

Start this job out as non-voting so that we can make sure it works as
expected before turning it on to be gating.

Change-Id: Ie99a88b95b28cee9443b05fd091f9e4f590e4cfb
Merge "Add gate-magnetodb-devstack job"