libceph: use CEPH_MON_PORT when the specified port is 0

Similar to userspace, don't bail with "parse_ips bad ip ..." if the
specified port is port 0, instead use port CEPH_MON_PORT (6789, the
default monitor port).

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>