sd: add inquireClusterLock method to StorageDomain

The leader version and the spm id may be maintained by the cluster
lock in some implementations (e.g. SANLock).
This patch introduces the capability of inquiring the cluster lock
for these values.

Change-Id: I10e64d74319ea6591a7edf8e17809d367a758386
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21426
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>