If a label is removed from the configuration while a node still
exists, the periodic cleanup performed by the metastatic driver
will raise an AttributeError exception when trying to access
the grace_time attribute, since the label pointer is None.
To address this, treat a missing label as if it has a grace time
of zero seconds.
This adds a test which simulates the issue.
This also adds an extra log entry for when a metastatic backing
node slot is deallocated is added so that we log both the allocation
and deallocation.
Change-Id: I0c104a2fe9874e2cd30e2bf2f2227569a73be243
1b1eab77b0