Enable ansible-lint Github action on every push.

By running ansible-lint we check if playbooks provided in
ansible-freipa follow Ansible's best practices, nd the verification
will be performed on every push (even on forks) or pull-request.

This patch provides the configuration needed to run ansible-lint
to the playbooks found in the `tests`, `playbooks` and `molecule`
directories, on every push or pull-request done on Github, using
Ansible's Github Action ansible/ansible-lint-action.

.ansible-lint

+exclude_paths:
+  - roles
+  - .tox
+  - .venv
+
+parseable: true
+
+quiet: false
+
+skip_list:
+  - '201'  # Trailing whitespace
+  - '204'  # Lines should be no longer than 160 chars
+  - '206'  # Variables should have spaces before and after: {{ var_name }}'
+  - '208'  # File permissions not mentioned
+  - '301'  # Commands should not change things if nothing needs doing'
+  - '305'  # Use shell only when shell functionality is required'
+  - '306'  # Shells that use pipes should set the pipefail option'
+  - '502'  # All tasks should be named
+  - '505'  # Referenced missing file
+
+use_default_rules: true
+
+verbosity: 1

.github/workflows/lint.yml

+---
+name: Run Linters
+on:
+  - push
+  - pull_request
+jobs:
+  linters:
+    name: Run Linters
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: "3.6"
+
+      - name: Run ansible-lint
+        uses: ansible/ansible-lint-action@master
+        with:
+          targets: |
+            tests/*.yml
+            tests/*/*.yml
+            tests/*/*/*.yml
+            playbooks/*.yml
+            playbooks/*/*.yml
+        env:
+          ANSIBLE_MODULE_UTILS: plugins/module_utils
+          ANSIBLE_LIBRARY: plugins/modules