Global only/except to make MR Pipelines user-friendly to use

Problem to Solve

With the https://gitlab.com/gitlab-org/gitlab-ce/issues/15310 we introduced a support for merge-requests pipelines. However, this implementation is not user-friendly as you have to manually specify for each job the valid only: syntax.

You can see a specific example of how difficult it can be to use only/except given some of the current behavior at gitlab-com/www-gitlab-com!17638 (merged); if we solve this correctly, solving that should be easier/possible.

Current defaults

Today's default only syntax:

rspec:
  only:
    - branches
    - tags

Enable MR pipelines:

In order to enable MR pipelines you should change it to look like:

rspec:
  only:
    - master
    - merge-requests
    - tags

Global only:

I think that we want to introduce a change that would allow to change the defaults of that specification as wide as possible, ideally on global context of job, and making it non-master specific:

only:
  - HEAD
  - merge-requests
  - tags

rspec: ...

We should also implement the HEAD.

Aliasing problem

The problem here is that only: can be a valid job name, so to make it possible we need to perform a filtering of potential jobs vs global configuration entries. I think that we could consider things to be potential jobs if they are hash containing script: entry. So technically be able to detect:

only:
  script: ...

only:
  - HEAD
  - merge-requests
  - tags

This is likely useful, as we want to introduce more global entries, but keeping it backward compatible. Today's global configuration entries do overwrite jobs under that name.