zuul: port the legacy multinode grenade job

Make it a native Zuul v3 job.
It is a simple variant of the base 'grenade-multinode' job
where oslo.versionedobjects is installed from source instead of pip.

Also use a slightly more common name by dropping the "-devstack"
component from the name (all grenade jobs are also devstack jobs).

Finally, remove the hardcoded xenial nodeset, thus inheriting the
default testing definition for it, and probably unbreaking
the job which has been failing so far.

Change-Id: Ia209311d57f607b2ddfcab8518264cd7f670dc8f