Add --dry-run option to undercloud install

This will help greatly with the development of the --use-heat path by
printing the underlying "undercloud deploy" command instead of
executing it. This may also help debug undercloud install issues in
the field.

Change-Id: I42ce297e5663ff782cfb6afa9058daf8abdf389b
Blueprint: containerized-undercloud

releasenotes/notes/undercloud-dry-run-30264c62d6d44626.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - The ``openstack undercloud install`` command now has a ``--dry-run``
+    argument which will print the resulting install command instead of
+    executing it.

tripleoclient/tests/v1/undercloud/test_undercloud.py

@@ -44,7 +44,7 @@ class TestUndercloudInstall(TestPluginV1):
         # DisplayCommandBase.take_action() returns two tuples
         self.cmd.take_action(parsed_args)
 
-        mock_subprocess.assert_called_with('instack-install-undercloud')
+        mock_subprocess.assert_called_with(['instack-install-undercloud'])
 
     @mock.patch('subprocess.check_call', autospec=True)
     def test_undercloud_install_with_heat(self, mock_subprocess):

tripleoclient/v1/undercloud.py

@@ -52,6 +52,13 @@ class InstallUndercloud(command.Command):
             default=False,
             help=_("Do not perform undercloud configuration validations"),
         )
+        parser.add_argument(
+            '--dry-run',
+            dest='dry_run',
+            action='store_true',
+            default=False,
+            help=_("Print the install command instead of running it"),
+        )
         return parser
 
     def take_action(self, parsed_args):
@@ -59,13 +66,15 @@ class InstallUndercloud(command.Command):
 
         utils.ensure_run_as_normal_user()
         if parsed_args.use_heat:
+            no_validations = parsed_args.dry_run or parsed_args.no_validations
             cmd = undercloud_config.\
-                prepare_undercloud_deploy(no_validations=parsed_args.
-                                          no_validations)
-            print("Running: %s" % ' '.join(cmd))
-            subprocess.check_call(cmd)
+                prepare_undercloud_deploy(no_validations=no_validations)
         else:
-            subprocess.check_call("instack-install-undercloud")
+            cmd = ["instack-install-undercloud"]
+        if parsed_args.dry_run:
+            print(' '.join(cmd))
+        else:
+            subprocess.check_call(cmd)
 
 
 class UpgradeUndercloud(InstallUndercloud):