Add test for 7188
Insert data during upgrade
Lower RF to 2. Increase write count.

Based on Aleksey's recommendations.
Restrict 1.2->2.0 upgrade tests to being run on 2.0 or higher
Add comments explaining upgrade path
Merge branch 'ccm_develop'
Merge branch 'master' of http://github.com/riptano/cassandra-dtest
Merge branch 'master' into 7188

Conflicts:
	upgrade_supercolumns_test.py
Stop/start nodes when upgrading the version
upgrade test, flush nodes before shutting down
Merge pull request #96 from riptano/7188

7188