Switch to using pbr version for cli version string

This commit switches from using the hard coded version strings in the
cliff main app to using the version string generated pbr. This means
that the version returned from --version on the tempest cli will
actually reflect the version of the code you're using.

Change-Id: I74b2a2c4f1cf9b4a6639c35545b3256ea94cd961
diff --git a/tempest/cmd/main.py b/tempest/cmd/main.py
index 762e982..577df9b 100644
--- a/tempest/cmd/main.py
+++ b/tempest/cmd/main.py
@@ -16,8 +16,7 @@
 
 from cliff import app
 from cliff import commandmanager
-
-TEMPEST_CLI_VERSION = '0.1'
+from pbr import version
 
 
 class Main(app.App):
@@ -27,7 +26,7 @@
     def __init__(self):
         super(Main, self).__init__(
             description='Tempest cli application',
-            version=TEMPEST_CLI_VERSION,
+            version=version.VersionInfo('tempest').version_string(),
             command_manager=commandmanager.CommandManager('tempest.cm'),
             )