Move terminology from "queue" to "pipeline".

The zuul terminology for queues was changed to pipelines to help clean
up the code and make it easier to read and debug. Update the
documentation to reflect this terminology change.

Change-Id: I8805f7ee201196b9658b8ea3546d4451aed4c700
Reviewed-on: https://review.openstack.org/13312
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/doc/source/gating.rst b/doc/source/gating.rst
index bf245dd..cf5964c 100644
--- a/doc/source/gating.rst
+++ b/doc/source/gating.rst
@@ -39,7 +39,7 @@
 developers to create changes at a rate faster than they can be tested
 and merged.
 
-Zuul's DependentQueueManager allows for parallel execution of test
+Zuul's DependentPipelineManager allows for parallel execution of test
 jobs for gating while ensuring changes are tested correctly, exactly
 as if they had been tested one at a time.  It does this by performing
 speculative execution of test jobs; it assumes that all jobs will