rbd: expose options available to rbd map

Add a -o / --options option, which would allow users to specify
rbd-specific and generic ceph client and osd options available at
mapping time in a comma separated list (similar to mount(8) mount
options).

Exposed options are:

- rw
- ro (equivalent to existing --read-only flag)
- share
- noshare
- crc
- nocrc
- osdkeepalive=%d
- osd_idle_ttl=%d

The rw/ro < 3.7 kernels compatibility kludge added in commit
fb0f1986449b is preserved.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>