Rehome base upgrade checks class

Class neutron.cmd.upgrade_checks.base.BaseChecks can be
used as base class for upgrade checks implemented in stadium
or 3rd party projects (e.g. in [1]).
It will be better to have it in neutron-lib rather than in
neutron.

[1] https://review.openstack.org/#/c/637203/

Change-Id: I6f9777358a33444fabd09e611a9cddd807827573

neutron_lib/utils/upgrade_checks.py

@@ -0,0 +1,36 @@
+# Copyright 2018 Red Hat Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import abc
+
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class BaseChecks(object):
+
+    """Base class providing upgrade checks.
+
+    Stadium projects which want to provide their own upgrade checks to
+    neutron-status CLI tool should inherit from this class.
+
+    Each check method have to accept neutron.cmd.status.Checker
+    class as an argument because all checkes will be run in context of
+    this class.
+    """
+
+    @abc.abstractmethod
+    def get_checks(self):
+        """Get tuple with check methods and check names to run."""
+        pass

releasenotes/notes/add-base-upgrade-checks-class-f6da1d501663d8c5.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Base class for upgrade checks used in checks in
+    ``neutron-status upgrade check`` tool is now available in
+    ``neutron_lib.utils.upgrade_checks`` and can be reused by other projects.