design/external-management.md says that we should allow specifying
Filesystems that have no pools. When this is done, the CRD should
interepret that as a request to just manage the MDS daemons without
creating a new filesystem.
Don't fail the verification if there are no data pools in the spec,
and just skip creating a new filesystem if that's the case.
Also, add a testcase for this usage.
Signed-off-by: Jeff Layton <jlayton@redhat.com>