Make CI to use the permission of the user who is trigger the build

This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5735, but with removed all LFS code that is added by: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6043.

This also incorporates most of LFS code added in !6043 to simplify further merge.

See merge request !6409