This adds parsing if the `secret_file` from YAML. And reads the
contents of the configured file.

If no file is configured, we fall back to the `.gitlab_shell_secret`
in the root dir of the configuration.

If the configured path does not start with a `/` we start looking for
the file relative to the root dir of the configuration.

This adds the possibility to enable features for GitLab shell.

The first feature being recognized is "Discover": It's the command
that is executed when running `ssh git@gitlab.example.com` and is
called without a command.

The gitlab key id or username is already parsed from the command line
arguments.

Currently we only support communicating with GitLab-rails using unix
sockets. So features will not be enabled if the GitLab-url is using a
different protocol. The url for this read from the config yaml.

Pending ruby-specs have been added for the gitlab-shell command.

Refactor to have separate command packages