gitlab/gitlab-shell-config.yml: Explicitly point it to secret file

Explicitly point gitlab-shell to location where we keep secrets.

We already pointeg gitlab to that place and now we do that for
gitlab-shell so those 2 peieces can connect to each other ok.

Regarding the setting itself - there is no such block in omnibus-gitlab,
but it is present in gitlab-shell configuration example:

    https://gitlab.com/gitlab-org/gitlab-shell/blob/82b3a4e8/config.yml.example#L35

/cc @kazuhiko, @jerome

software/gitlab/instance-gitlab.cfg.in

@@ -270,7 +270,7 @@ update-command =
     ${:tune-command}
 
 
-# NOTE there is no need to link/create .gitlab_shell_secret - we will set path to it
+# NOTE there is no need to link/create .gitlab_shell_secret - we set path to it
 # in gitlab & gitlab-shell configs, and gitlab creates it on its first start
 [gitlab-work]
 <= work-base

software/gitlab/template/gitlab-shell-config.yml.in

@@ -30,6 +30,11 @@ repos_path: "{{ gitlab.repositories }}"
 # NOTE not used in slapos version (all access via https only)
 auth_file: "{{ gitlab.var }}/sshkeys-notused"
 
+# File that contains the secret key for verifying access to GitLab.
+# Default is .gitlab_shell_secret in the root directory.
+secret_file: "{{ gitlab_shell.secret }}"
+
+
 # Redis settings used for pushing commit notices to gitlab
 redis:
   bin: {{ redis_binprefix }}/redis-cli