mon: Monitor: change 'status' function names for clarity's sake

I found myself going through status functions this morning and realizing
just how annoyingly confusing the names are.  We have a '_mon_status()'
function and a 'get_status()' function.  Just by looking at them we figure out
just what each one's intended purpose is, but it's annoying to have to check
them out.  So I just changed them to something that is reasonable and clear.

'_mon_status()' thus becomes 'get_mon_status()', as it obtains mon-specific
status about the mon we are poking.

'get_status()' becomes 'get_cluster_status()', as it reports on cluster status
instead of 'just-monitor-specific status'.

This is not in any way a critical fix.  Just function renaming.  For the sake of
humanity.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>