Move orphaned unit tests to cmdlib/cluster_unittest.py

Move orphaned unit tests to cmdlib/cluster_unittest.py

Through some merge from 2.9 to master, some unit tests
in the file ganeti.comdlib.common_unittest.py were
no longer referenced in the Makefile and as such not
run anymore. With the introduction of the cmdlib test
suite and the following extension of the unit tests,
the orphaned unit tests (which are not already subsumed
by others) are moved into the new unit test files.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>