Updated rally gate scenarios

Change-Id: I021e4be45cf3e9d92c9e099cec63ae844831c995
Refactored test_images.py and images.py

Refactored test_images.py to include separate tests for each
Method in the Context Class

Also removed default values of arguments to image context class in
images.py which are now mandatory

Change-Id: Iaa510ea10b6f54eb0968382e8ea648fe4dcc288d
Merge "Updated rally gate scenarios"
Merge "Refactored test_images.py and images.py"
Periodic runner refactoring

  'preiodic' runner renamed to rps (runs per second).
  Extensible pool of multiprocessing.Process was implemented instead
  of using constant length multiprocessing.Pool. That should decrease memory
  usage on large iterations and related errors.

Change-Id: I6e367ea30d89d5d0e7ba212ed22e8658421ac9bd