Restore unauthenticated access to public container registries

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24284

See merge request !2025
Signed-off-by: NRémy Coutable <remy@rymai.me>

Use a permissions of user to access all dependent projects from CI jobs (this also includes a container images, and in future LFS files)

This reverts commit 13e37a3e.

Auth.find was a very generic name for a very specific method.
Auth.find_in_gitlab_or_ldap was inaccurate in GitLab EE where it also
looks in Kerberos.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb