Include "OpenStack" string in the user agent

EGI's accounting team requires that we put "OpenStack" in the UA string.

closes #38

Author: alvarolopez

caso/__init__.py

@@ -20,4 +20,4 @@
 __version__ = pbr.version.VersionInfo(
     'caso').version_string()
 
-user_agent = "caso/%s" % __version__
+user_agent = "caso/%s (OpenStack)" % __version__