Fix package manager used during undercloud packages update

As described in the related bug we need to use dnf instead of yum
for centos8 otherwise it fails in the undercloud packages update.

I am using the python version to determine - py3 is dnf. As part of
discussion here when dnf isn't available fall back to yum.

Found as part of the work in [1].

Related-Bug: 1886837
[1] https://tree.taiga.io/project/tripleo-ci-board/task/1817

Change-Id: Idac62d37a19ee49f30936e20021a9dab5af40eec

tripleoclient/tests/v1/undercloud/test_install_upgrade.py

@@ -575,6 +575,8 @@ class TestUndercloudUpgrade(TestPluginV1):
         app_args.verbose_level = 1
         self.cmd = undercloud.UpgradeUndercloud(self.app, app_args)
 
+    @mock.patch('os.system')
+    @mock.patch('sys.version_info')
     @mock.patch('tripleoclient.utils.prompt_user_for_confirmation',
                 return_value=True)
     @mock.patch.object(sys, 'executable', 'python2')
@@ -587,14 +589,30 @@ class TestUndercloudUpgrade(TestPluginV1):
     @mock.patch('subprocess.check_call', autospec=True)
     @mock.patch('tripleoclient.utils.run_command', autospec=True)
     def test_undercloud_upgrade_default(self, mock_run_command,
-                                        mock_subprocess,
-                                        mock_wr,
-                                        mock_os, mock_copy, mock_user,
-                                        mock_getuid, mock_confirm):
+                                        mock_subprocess, mock_wr,
+                                        mock_os_mkdir, mock_copy, mock_user,
+                                        mock_getuid, mock_confirm, mock_sys,
+                                        mock_os_sys):
         arglist = ['--no-validations']
         verifylist = []
         parsed_args = self.check_parser(self.cmd, arglist, verifylist)
 
+        mock_sys.major = 3
+        mock_os_sys.return_value = 0
+        # DisplayCommandBase.take_action() returns two tuples
+        self.cmd.take_action(parsed_args)
+        mock_run_command.assert_called_with(
+            ['sudo', 'dnf', 'upgrade', '-y',
+             'python2-tripleoclient',
+             'openstack-tripleo-common',
+             'openstack-tripleo-heat-templates',
+             'openstack-tripleo-validations',
+             'tripleo-ansible'],
+            name='Update extra packages'
+        )
+        mock_os_sys.assert_called_with("which dnf")
+
+        mock_sys.major = 2
         # DisplayCommandBase.take_action() returns two tuples
         self.cmd.take_action(parsed_args)
         mock_run_command.assert_called_with(
@@ -606,6 +624,7 @@ class TestUndercloudUpgrade(TestPluginV1):
              'tripleo-ansible'],
             name='Update extra packages'
         )
+
         mock_subprocess.assert_called_with([
             'openstack', 'undercloud', 'upgrade', '--skip-package-updates',
             '--no-validations'])

tripleoclient/v1/undercloud.py

@@ -183,7 +183,11 @@ class UpgradeUndercloud(InstallUndercloud):
         if not packages:
             return
 
-        cmd = ['sudo', 'yum', 'upgrade', '-y'] + packages
+        pkg_manager = 'yum'
+        if sys.version_info.major >= 3 and os.system('which dnf') == 0:
+            pkg_manager = 'dnf'
+
+        cmd = ['sudo', pkg_manager, 'upgrade', '-y'] + packages
 
         if not dry_run:
             self.log.warning("Updating necessary packages: {}".format(