Add offline mode for skipping requirements installation (#1319)

Fixes: #930

.ansible-lint

@@ -34,3 +34,6 @@ warn_list:
   - skip_this_tag
   - git-latest
   - experimetal  # experimental is included in the implicit list
+
+# offline mode disables installation of requirements.yml
+offline: false

src/ansiblelint/_prerun.py

@@ -40,8 +40,8 @@ def check_ansible_presence() -> None:
 
 
 def prepare_environment() -> None:
-    """Make custom modules available if needed."""
-    if os.path.exists("requirements.yml"):
+    """Make dependencies available if needed."""
+    if not options.offline and os.path.exists("requirements.yml"):
 
         cmd = [
             "ansible-galaxy",

src/ansiblelint/cli.py

@@ -247,6 +247,13 @@ def get_cli_parser() -> argparse.ArgumentParser:
         dest='config_file',
         help='Specify configuration file to use.  ' 'Defaults to ".ansible-lint"',
     )
+    parser.add_argument(
+        '--offline',
+        dest='offline',
+        action='store_const',
+        const=True,
+        help='Disable installation of requirements.yml',
+    )
     parser.add_argument(
         '--version',
         action='store_true',
@@ -269,6 +276,7 @@ def merge_config(file_config, cli_config: Namespace) -> Namespace:
         'quiet',
         'use_default_rules',
         'progressive',
+        'offline',
     )
     # maps lists to their default config values
     lists_map = {

src/ansiblelint/config.py

@@ -49,6 +49,7 @@
     mock_modules=[],
     mock_roles=[],
     loop_var_prefix=None,
+    offline=False,
 )
 
 # Used to store detected tag deprecations