gitlab_runners inventory plugin: runtime en var and list default behavior (#611)

* chore: runtime en var and list default bahevaior 

Ability to pick options values from env vars on gitlab_runners inventory plugin
Remove default 20 items limit to runners list on gitlab_runners inventory plugin

* Changelog fragment

Co-authored-by: Felix Fontein <felix@fontein.de>

* Changelog fragment

* Badly placed fragment

Co-authored-by: Felix Fontein <felix@fontein.de>

* changelog fragment for api token

* changelog fragment

* Update changelogs/fragments/611-gitlab-runners-env-vars-intput-and-default-item-limit.yaml

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update changelogs/fragments/611-gitlab-runners-env-vars-intput-and-default-item-limit.yaml

Co-authored-by: Andrew Klychkov <aaklychkov@mail.ru>

* fix: remove default filter all due to  #440

* fix: remove default filter all due to  #440

* chore: add os env var for filter input

* chore: add os env var for filter input

Co-authored-by: Felix Fontein <felix@fontein.de>
Co-authored-by: Andrew Klychkov <aaklychkov@mail.ru>

changelogs/fragments/611-gitlab-runners-env-vars-intput-and-default-item-limit.yaml

@@ -0,0 +1,2 @@
+minor_changes:
+  - gitlab_runners inventory plugin - permit environment variable input for ``server_url``, ``api_token`` and ``filter`` options (https://github.com/ansible-collections/community.general/pull/611).

plugins/inventory/gitlab_runners.py

@@ -30,17 +30,26 @@ DOCUMENTATION = '''
               - gitlab_runners
         server_url:
             description: The URL of the GitLab server, with protocol (i.e. http or https).
+            env:
+              - name: GITLAB_SERVER_URL
+                version_added: 1.0.0
             type: str
             required: true
             default: https://gitlab.com
         api_token:
             description: GitLab token for logging in.
+            env:
+              - name: GITLAB_API_TOKEN
+                version_added: 1.0.0
             type: str
             aliases:
               - private_token
               - access_token
         filter:
             description: filter runners from GitLab API
+            env:
+              - name: GITLAB_FILTER
+                version_added: 1.0.0
             type: str
             choices: ['active', 'paused', 'online', 'specific', 'shared']
         verbose_output: