Extra checks:
- Check that only tags=['common', 'validate',
'pre-upgrade'] are accepted.
- Fail if tags not defined.
- Fail if 'step|int == ' condition inside 'when'
is not the evaluated first.
- Suggest the use of lists to append conditions
inside 'when'.
Change-Id: I15f6d4cb6f2a13d04580779a93a02daf86f8b412