[hbck] Fix intermittent failures on TestHBaseFsck.testHBaseFsck

Its seems that on the 0.92 branch in particular, TestHBaseFsck.testHBaseFsck is intermittently failing.

In the test, a region's assignment is purposely changed in META but not in ZK. After the equivalent of 'hbck -fix', a subsequent check that should be clean comes up with a new ZK assignment but with META still being inconsistent with ZK. The RS in ZK sometimes this points to the same RS, but sometimes it "moves" to another ZK.