Add cancel functionality to the REST API

[Baguage]

Moved from redmine, ticket 6364
How to implement as URL? Possible options are:

• PATCH with body = { action : cancel }
• POST to /api/v1/sim-groups/{id}/cancel/
• DELETE HTTP method
  ** This may not be appropriate because we don't want to delete the simulation or group resource. We just want to stop its execution.

Travis CI's API uses:

Cancel Job
POST /jobs/{job.id}/cancel
This request always needs to be authenticated.