Adding api_version to FakeApp

In order to use app.api_version['<some_service>'] we need to have the
api_version on FakeApp otherwise the test will fail.

Patch that will benefit from this:
https://review.openstack.org/#/c/572536/

Change-Id: Ibb3c548daf2a62b6c4aefb5e257372dd5c56521e

openstackclient/tests/unit/fakes.py

@@ -106,6 +106,7 @@ class FakeApp(object):
     def __init__(self, _stdout, _log):
         self.stdout = _stdout
         self.client_manager = None
+        self.api_version = {}
         self.stdin = sys.stdin
         self.stdout = _stdout or sys.stdout
         self.stderr = sys.stderr