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: I844a353bdfc1dde6f068a879002d16429d304e65
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>