Remove unused arguments

[#65335102]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Pull guard into controller that cares most about it

[#65335102]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Add /apps/:app_guid/restages endpoint

- The API has been bumped to 2.3.0
- POST; empty body is OK
- Triggers restaging an already pushed app
- Behaves like `cf push` (without uploading the app bits) which means there is
  a race condition when staging completes before all instances shut down,
  potentially causing mixed droplet versions in production.
- Is synchronous.

[#65335102]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>