Pass gitlabshell config through to gitlab-shell from gitaly

!2066 (merged) broke in staging because gitlab-shell saw that HTTPSettings was non-empty, and used empty values inside of it.

But now that !2104 (merged) is merged, we have protection on the ruby side against empty values.

This MR restores the original change from !2066 (merged), and also adds better test coverage for passing the config values into gitlab-shell.