Add CI_ENVIRONMENT_RUNNING variable

Description

CI_ENVIRONMENT_RUNNING will be set only when the environment (ie review/whatever) is running, it will allow to base conditions in jobs on the fact if the environment is already running.

It will be even better when https://gitlab.com/gitlab-org/gitlab-ce/issues/37397 is done - we can do things like have task which will only run when environment is already running - it's useful to have an auto-deploy to review app only if environment for that branch is already running.

job:
  only:
   expressions:
    - $CI_ENVIRONMENT_RUNNING

Proposal

Links / references