mon: erasure code pool properties defaults

If no properties are set when creating an erasure coded pool, default to
using the jerasure plugin with the cauchy_good technique which is the
fastest.

The defaults are set with osd_pool_default_erasure_code_properties.

The erasure code plugins are loaded from the directory specified in the
erasure-code-directory property. Contrary to the other properties it
will most commonly be the same throughout the cluster. The default is
set to /usr/lib/ceph/erasure-code with
osd_pool_default_erasure_code_directory

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