Define the default value for only/except policies

What does this MR do?

This is a preliminary work for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217#note_121796129.

Currently, if a job does not have only/except policies, the policy is considered as an unspecified state, and therefore the job is executed regardless of how it's executed and which branch/tags are targetted.

Ideally, this should be specified as only: ['branches', 'tags'], as it indicates that unspecified policy jobs are meant to run on any git references.

What are the relevant issue numbers?

• Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217#note_121796129
• Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/15310

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer