ceph: add support for --admin-socket <path>

Signed-off-by: Sage Weil <sage@inktank.com>
ceph: 'ceph daemon <name|path> <command ...>'

This is *much* easier than 'ceph --admin-daemon <path> <command ...>'.

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