mon: OSDMonitor: 'osd pool' - if we can set it, we must be able to get it

Add support to get the values for the following variables:
 - target_max_objects
 - target_max_bytes
 - cache_target_dirty_ratio
 - cache_target_full_ratio
 - cache_min_flush_age
 - cache_min_evict_age

``Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>``
add tests for cache_min_flush_age and cache_min_evict_age also ?