mon: unit test for osd pool create

mon: unit test for osd pool create

It is inconvenient to run such tests in the
qa/workunits/cephtool/test.sh because they require that the mon is
restarted to test errors in the format of the default erasure code
properties and check the appropriate error message is output.

osd-pool-create.sh runs a single mon from sources using command
line options and a temporary directory, the same way vstart.sh does but
lightweight.

Signed-off-by: Loic Dachary <loic@dachary.org>