Fix some inconsistency in docstrings

Changed docstring to ":returns:" according to
http://docs.openstack.org/developer/hacking/#docstrings

Change-Id: I531eeea4384ddb55e7d5e1e001356e4c6e5f272e
diff --git a/tempest/common/compute.py b/tempest/common/compute.py
index 6fc3843..5a14fbe 100644
--- a/tempest/common/compute.py
+++ b/tempest/common/compute.py
@@ -43,7 +43,7 @@
     :param wait_until: Server status to wait for the server to reach after
     its creation.
     :param volume_backed: Whether the instance is volume backed or not.
-    :returns a tuple
+    :returns: a tuple
     """
 
     # TODO(jlanoux) add support of wait_until PINGABLE/SSHABLE