rbd: expose mount_timeout map option

Expose mount_timeout map option.  (I missed it in commit 9b7364d2450c,
which added -o / --options option and among other options exposed
osdkeepalive and osd_idle_ttl timeouts.)

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1109 from ceph/wip-rbd-mount_timeout

rbd: expose mount_timeout map option

Reviewed-by: Sage Weil <sage@inktank.com>