Introduce --default-runtime parameter

Default to 'docker', --default-runtime will allow us to change the
runtime (docker, podman, etc) in the future.
In future patches, we'll add support for LibPod and this patch will
allow to start running "paunch apply" for a specific runtime backend.

Change-Id: I4660f37a3fc231830f4f3a875f63473d68ca7cfc

paunch/__init__.py

@@ -25,7 +25,8 @@
 LOG = logging.getLogger(__name__)
 
 
-def apply(config_id, config, managed_by, labels=None, docker_cmd=None):
+def apply(config_id, config, managed_by, labels=None, docker_cmd=None,
+          default_runtime='docker'):
     """Execute supplied container configuration.
 
     :param str config_id: Unique config ID, should not be re-used until any
@@ -38,19 +39,25 @@ def apply(config_id, config, managed_by, labels=None, docker_cmd=None):
     :param dict labels: Optional keys/values of labels to apply to containers
                         created with this invocation.
     :param str docker_cmd: Optional override to the docker command to run.
+    :param str default_runtime: Optional override to the default runtime used
+                                for containers.
 
     :returns (list, list, int) lists of stdout and stderr for each execution,
                                and a single return code representing the
                                overall success of the apply.
     :rtype: tuple
     """
+    # TODO(emilien) Call the right runner based on default_runtime
     r = runner.DockerRunner(managed_by, docker_cmd=docker_cmd)
-    builder = compose1.ComposeV1Builder(
-        config_id=config_id,
-        config=config,
-        runner=r,
-        labels=labels
-    )
+    if default_runtime == 'docker':
+        builder = compose1.ComposeV1Builder(
+            config_id=config_id,
+            config=config,
+            runner=r,
+            labels=labels
+        )
+    else:
+        LOG.error("container runtime not supported: %s" % default_runtime)
     return builder.apply()
 
 

paunch/cmd.py

@@ -56,6 +56,12 @@ def get_parser(self, prog_name):
             required=True,
             help=('ID to assign to containers'),
         )
+        parser.add_argument(
+            '--default-runtime',
+            dest='default_runtime',
+            default='docker',
+            help=('Default runtime for containers. Can be docker or podman.'),
+        )
         return parser
 
     def take_action(self, parsed_args):
@@ -72,7 +78,8 @@ def take_action(self, parsed_args):
             parsed_args.config_id,
             config,
             managed_by='paunch',
-            labels=labels
+            labels=labels,
+            default_runtime=parsed_args.default_runtime
         )
 
         return rc