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>
qa/workunits: cephtool: test for 'osd pool {get,set}-quota'

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #2029 from ceph/wip-8523

mon: OSDMonitor: add 'osd pool get-quota' command

Reviewed-by: Sage Weil <sage@redhat.com>