Allow gitlabshell config to be passed in through a config variable

Currently, the hooks part of gitlab shell functionality is part of gitaly under ruby/gitlab-shell. For historical reasons, this code uses configs that live under ruby/gitlab-shell/config.yml. This means gitaly configs live across two separate files, which is not ideal. Additionally, we are moving towards migrating the hooks into gitaly and eventually porting everything to Go.

In order to prepare for this transition, the first step is to move the relevant configs from ruby/gitlab-shell/config.yml into gitaly's config.toml.

To safely deprecate ruby/gitlab-shell/config.yml, we will pass in the gitlab shell config values through an environment variable GITALY_GITLAB_SHELL_CONFIG which is a json-encoded string of all the gitlab shell config values. But in order to keep this MR relatively small, those changes will be included in a separate MR.

This MR will only include the ruby changes.

addresses: #2182 (closed)