Refine PGs/OSD warning

[pponnuvel]

Currently ceph plugin produces warnings like:

    CephCrushErrors:
      - Found some Ceph osd(s) with > 500 pgs - this is close to the hard limit at
        which point they will stop creating pgs and fail - please investigate.

But this is based on default settings. When osd_max_pg_per_osd_hard_ratio and/or mon_max_pg_per_osd are configured differently, this check can (and does) produce a false positive.

So the plugin needs to take these two parameters into account calculating the optimal range.