mon: OSDMonitor: add 'osd pool get-quota' command

Enables us to obtain current quotas for a given pool.

Fixes: #8523

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>

Please note that the patch is not accompanied with a cephtool unit test.  I will add a test to the patch series once the cephtool/test.sh refactor branch is merged, as I simply don't want to deal with conflicts when either of the branches get merged.