Add pass and unstablerobot thresholds for stage3

This change adds two parameters to the stage3 Jenkinsfile in
ci_stage3_groovy:

- ROBOT_PASS_THRESHOLD: % passed Robot tests to mark the build as
  passed
- ROBOT_UNSTABLE_THRESHOLD: % passed Robot tests to mark the build
  as unstable

These two parameters are passed to robotPublisher, the class in Robot
Framework Jenkins plugin to publish test results.

Change-Id: Ia2abdf996033163572836afe88cfa0675683fcb1
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>