CQL3 shouldn't lowercase DC names for NTS

patch by slebresne; reviewed by jbellis for CASSANDRA-5292
Remove assertion that gossip is active in migration manager.
Patch by brandonwilliams, reviewed by iamaleksey for CASSANDRA-5236
Merge branch 'cassandra-1.1' into cassandra-1.2
Merge branch 'cassandra-1.2' into trunk