Implement rescue mode support for Rackspace

Create real and mock rescue and unrescue methods for Rackspace compute V2
Create tests for rescue and unrescue methods
Rectify issues with tests and status

* Since the server will remain in ACTIVE state for up to a few minutes, do not
  set the status (to prevent a situation where the state bounces between
  PREP_RESCUE and ACTIVE).
* Add format() test for the call to #rescue
* Remove extraneous sleep from the test against #unrescue
* Remove immediate tests for state.  Rather, depend upon the wait_for function.
Merge pull request #1753 from cwuest/master

[Rackspace | ComputeV2] Add calls to enter and exit rescue mode