Switch standalone & undercloud to Podman by default

Unless an operator overrides it, the default undercloud & standalone
deployment will be on Podman and not on Docker anymore.

blueprint podman-support
Change-Id: Ided7b88fa984ca49f487fd0573e0c5e78c5480e8

tripleoclient/config/standalone.py

@@ -194,7 +194,7 @@ class StandaloneConfig(BaseConfig):
                                '/etc/sysconfig/docker.')
                         ),
             cfg.StrOpt('container_cli',
-                       default='docker',
+                       default='podman',
                        help=_('Container CLI used for deployment; '
                               'Can be docker or podman.')),
         ]