ceph_mon: check for existing mon store before opening db

This will make sure that the monitor data directory is populated before
proceeding to opening it up.  We have to do this in order to work around
leveldb, which will create 'store.db' (even though it won't populate it)
upon opening the database.  Unwillingly, running the monitor without
first issuing a mkfs would have us ending up with a 'store.db' in the
monitor's data directory, and later on we would get errors from the
monitor not finding magic values within the store -- which given mkfs
hadn't been run isn't surprising.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
a good pattern is to make functions named as a predicate (foo_exists, is_foo, has_bar) return true/false, and have declaritive(?) names (do_x, check_y) return 0 success or negative error.  Maybe rename this check_mon_data_exists() ?