#7317 Tests for ENOSPC and OSD epoch barrier handling

Depends on:
https://github.com/ceph/ceph/pull/2836

The general assumption here is that the test is in control of the cluster and nothing is messing with the RADOS cluster out of band.  Things like e.g. the thrasher will alway be unsafe to run against this test because they would mutate the fullness of the cluster unpredictably.

However, it's harmless to turn these particular changes into GreaterEqual rather than Equal, to be a bit more flexible in the face of e.g. anything that e.g. updates the osd map during initialisation in the future.  The later checks for mount_a's map having *not* updated still have to be Equal checks as we are checking that it isn't picking up new maps (in order to check that mount_b picking up new maps is really a result of what we're explicitly doing).

tl;dr; done :-)