spec: bump sanlock version to 2.8

The new SANLock inquireClusterLock method relies on two new API
introduced in sanlock 2.8:

  sanlock.read_resource(path, offset=0) -> dict
  sanlock.read_resource_owners(lockspace, resource, disks) -> list

Change-Id: Ib733c5231a475ac1c5c648f55d77ffc4fa752f32
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23552
Reviewed-by: Dan Kenigsberg <danken@redhat.com>