gitlab-ci.yml: Replace template anchors with extends

gitlab-ci.yml: Replace template anchors with extends

'extends' is slightly more readable and definitely more flexible in
terms of allowing includes of templates.
The main reason for this patch though is that the next patch converts
the 'only/except' syntax to the new (preferable) 'rules' syntax.
Variable anchors are still kept intact because the use case there is
different from regular template anchors.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>