vfs_ceph: add support to select ceph file system

vfs_ceph: add support to select ceph file system

Add a config parameter `ceph:filesystem` that will be passed to
ceph_select_filesystem when provided. This allows shares on a single
smbd to access multiple different cephfs file systems.

The ceph_select_filesystem call was added as part of ceph 'nautilus'
(v14), released on 2019-03-19 and EOLed on 2021-06-30.
Since ceph 'pacific' (v16) multiple file system support is stable
and ready to use. At the time of this commit, 'pacific' is the oldest
actively supported version of ceph upstream.

Since samba tests building on ubntu 18.04, which has ceph packages
older than v14, a waf check for the function is added to test for
the presence of ceph_select_filesystem and disable its use on
these very old platforms.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
Reviewed-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Anoop C S <anoopcs@samba.org>