Support negative matches in .gitlab-ci.yml

What does this issue do?

This adds support in only/except:variables: support for != and !~ operators.

This is the most naive approach. For example:

($AA == "AAA") && ($BB == "CCC") && ($DD == "XXX")

except:
  variables:
    - $AA != "AAA"
    - $BB != "CCC"
    - $DD != "XXX"