ceph-mon: override 'leveldb_*' config options for the monitor

ceph-mon: override 'leveldb_*' config options for the monitor

'leveldb_*' options are currently used both by the monitor and the osd.
However, the monitor has quite different requirements from those of the
osds.

We need to specify some default values that must squash the defaults we
have for 'leveldb_*' options, while allowing users to overriding them too.
We take this not-exactly-ideal-but-still-good-enough approach of
defining the monitor-specific defaults in the 'default arguments' to
global_init(), thus allowing the user's options to take precedence over
whatever we define.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>