Add Job Variable for user associated with CI_JOB_TOKEN

Description

Currently we have CI_JOB_TOKEN, CI_REGISTRY_USER and CI_REGISTRY_PASSWORD. CI_REGISTRY_PASSWORD or CI_JOB_TOKEN can both be used together with CI_REGISTRY_USER to authenticate against the registry. The problem is the naming of the CI_REGISTRY_USER variable implies it's only suitable for authenticating against the registry, and nothing else.

The new tip introduced in gitlab-ce!10221 will inform users about the possibility of authenticating against repositories as well, but there is no job variable associated with the user for this kind of authentication.

Proposal

Add a new CI job variable for authenticating against repositories, or rename the CI_REGISTRY_USER to sound less intimidating for uses other than authenticating against the registry.

Links / references

The predefined job variables