Enable 'make mockrun' to actually run.

'make mockrun' fails when *scheduler* and *mock* is set in the config.
This is because the scheduler setup requires that *cassandra_cluster*
has been set before hand, and this does not happen when running with
'--mock'.
Test related to 'make mockrun'

Ensure that the scheduler is only setup when the backend is not mocked.
Merge pull request #319 from rackerlabs/fix-make-mockrun

Fix make mockrun.