Update version.version_string to actually be a string

pbr provides VersionInfo.version_string() as a method to determine
the version.  mistral.version.version_string can hide this and
provide a uniform string interface to config and launch.

This fixes a problem with mistral-server --version generating
an exception because the version passed to argparse was a method
instead of a string.

Change-Id: Ie468685e4360bfaec5d82b02f8cf1a27a93bcd94
Closes-Bug: 1796921

mistral/cmd/launch.py

@@ -143,7 +143,7 @@ MISTRAL_TITLE = """
 ||      || || __//   ||_// ||      \\\__// \\\_ ||
 
 Mistral Workflow Service, version %s
-""" % version.version_string()
+""" % version.version_string
 
 
 def print_server_info():

mistral/tests/unit/test_version.py

@@ -0,0 +1,19 @@
+#    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.
+
+from mistral.tests.unit import base
+from mistral.version import version_string
+
+
+class VersionStringTest(base.BaseTest):
+    def test_version(self):
+        self.assertIsInstance(version_string, str)

mistral/version.py

@@ -15,4 +15,4 @@
 from pbr import version
 
 version_info = version.VersionInfo('mistral')
-version_string = version_info.version_string
+version_string = version_info.version_string()