specify the runner's tags explicitly

We need a distinction between the executor name and the tags. This is
important to work with pre-existing CI configurations, without needing
to modify them. It also makes sense to seperate this in general as a tag
may not relate directly to the executor.

For example:

executor= docker
tag= linux

roles/gitlab-runner/tasks/main.yml

@@ -31,7 +31,7 @@
       name: "{{ ansible_fqdn }}"
       description: "virtual machine on {{ ansible_fqdn }}"
       token: "{{ ansible_local.gitlab_runner.registration_token }}"
-      tag_list: "{{ ansible_local.gitlab_runner.executor }}"
+      tag_list: "{{ ansible_local.gitlab_runner.tags }}"
     status_code: 201
     return_content: yes
   register: runner_token

roles/gitlab-runner/templates/gitlab_runner.fact

@@ -2,5 +2,6 @@
   "registration_token": "{{ ansible_local.gitlab_runner.registration_token }}",
   "runner_token": "{{ runner_token.content | from_json | json_query('token') }}",
   "gitlab_domain": "{{ ansible_local.gitlab_runner.gitlab_domain }}",
-  "executor": "{{ ansible_local.gitlab_runner.executor }}"
+  "executor": "{{ ansible_local.gitlab_runner.executor }}",
+  "tags": "{{ ansible_local.gitlab_runner.tags }}"
 }