Add the skeleton of a flocker-changestate command (fixes #196)

Here we've:
 * added the skeleton of a flocker-changestate command
 * added tests for the command line options
 * including catching YAML parsing errors and reporting those on stderr as UsageErrors.

Build: http://build.flocker.hybridcluster.net/boxes-flocker?branch=change-local-state-takes-yml-196
Fixes #196