LU-2494 tests: add interop support to framework check

Modify the test-framework wait_mds_ost_sync() function to work
with mds_sync when old_sync_processed doesn't exist.
This should cover both old and new servers.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I2e5c430a96ac40a1ca87990696e7daaad5a69102
Reviewed-on: http://review.whamcloud.com/5021
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>