Feature 11001: Robot framework linting for E2E tests

This change includes robotframework-lint and robocop checks which will
improve code maintenance.

In addition, all references to ROBOT_DEVOPS_FOLDER have been properly
replaced in all testsuites.

Change-Id: I6efb306b51302d0400240bcd57435f0990021415
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>