Don't run pipelines for tags

This changes the CI config to only run for:

• master

• merge requests

• when $RELEASE is set (from triggered pipelines)

• Control pipeline: https://gitlab.com/gitlab-org/gitlab-qa/pipelines/114627546

• New pipeline: https://gitlab.com/gitlab-org/gitlab-qa/pipelines/118847998 (ce:repository_storage, ce:repository_storage-quarantine, ee:repository_storage, and ee:repository_storage-quarantine were added in the meantime).

• Triggered CE pipeline: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/pipelines/118949593 => https://gitlab.com/gitlab-org/gitlab-qa/pipelines/118982148

• Triggered EE pipeline: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/pipelines/118952916 => https://gitlab.com/gitlab-org/gitlab-qa/pipelines/118985664

Closes #533 (closed).