Write gitlab shell config from gitaly to gitlab shell as env var

!2053 (merged) modified gitlab-shell to take GITALY_GITLAB_SHELL_CONFIG as a json encoded string env var.

This MR does the work of modifying the config, and passing these values through to gitlab-shell from the git-receive-pack RPCs. It also updates tests.

addresses: #2182 (closed)

NOTE: a separate MR is necessary for passing the config to gitlab-shell from the services/operations RPCs