Rename to Constants.py

Rename to Constants.py

Motivation: Make the constants available also for Robot.
Restraint: Robotframework user guide says:
  Because variable files are always imported using a file system path,
  creating them as classes has some restrictions:
    Python classes must have the same name as the module they are located.

Change-Id: I638ef3fe045db132e366de2e2699638b8637e45e
Signed-off-by: Vratko Polak <vrpolak@cisco.com>