mon: updated for logging and additional metrics collected for rbd and osd hosts

mon: updated for logging and additional metrics collected for rbd and osd hosts

collector class now scans the cluster to determine the count of RBDs. Each monitor
will pick a discrete set of pools to scan, so the overall load is shared across monitors.

In addition, since the osd tree command is used to determine the up/down state of
the OSDs, the same output is used to determine the number of osd hosts in the
configuration. Prior to this change the determination was inferred through a
graphite query.