Run jobs only when they are run by a scheduled pipeline

Description

With the new scheduled pipelines feature jobs might be run only when they are are run by a scheduled pipeline.
This might be useful for heavy jobs that should not be triggered by a push.

Proposal

Add scheduled to the list of keywords for the when keyword.

Links / references

https://docs.gitlab.com/ee/ci/yaml/#when

Documentation blurb

At https://docs.gitlab.com/ee/ci/yaml/#when we can add item number 5: scheduled - execute job only when scheduled in a scheduled pipeline