ceph.in: --admin-daemon was not returning EINVAL on bad command

Fix by restructuring code to hoist common code and have only one
place where admin_socket is actually called.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #506 from dmick/wip-admin-daemon

Reviewed-by: Sage Weil <sage@inktank.com>