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
diff --git a/tempest/common/fixed_network.py b/tempest/common/fixed_network.py
index 1928a22..56cd331 100644
--- a/tempest/common/fixed_network.py
+++ b/tempest/common/fixed_network.py
@@ -83,7 +83,7 @@
            is the network to be used, and it's not visible to the tenant
     :param shared_network_name: name of the shared network to be used if no
            tenant network is available in the creds provider
-    :return a dict with 'id' and 'name' of the network
+    :returns: a dict with 'id' and 'name' of the network
     """
     caller = misc_utils.find_test_caller()
     net_creds = creds_provider.get_primary_creds()
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 147c0ba..a5ed645 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -694,8 +694,8 @@
         def cidr_in_use(cidr, tenant_id):
             """Check cidr existence
 
-            :return True if subnet with cidr already exist in tenant
-                False else
+            :returns: True if subnet with cidr already exist in tenant
+                  False else
             """
             cidr_in_use = self._list_subnets(tenant_id=tenant_id, cidr=cidr)
             return len(cidr_in_use) != 0
diff --git a/tempest/test.py b/tempest/test.py
index a9c8ba7..435d10a 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -495,7 +495,7 @@
         :param credential_type: string - primary, alt or admin
         :param roles: list of roles
 
-        :returns the created client manager
+        :returns: the created client manager
         :raises skipException: if the requested credentials are not available
         """
         if all([roles, credential_type]):