mon: OSDMonitor: add "osd pool get <pool> erasure_code_profile" command

Enable us to obtain the erasure-code-profile for a given erasure-pool.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Signed-off-by: Sage Weil <sage@inktank.com>
qa/workunits/cephtool/test.sh: test osd pool get erasure_code_profile

Signed-off-by: Sage Weil <sage@inktank.com>