Support multiple triggers

Add the ability for Zuul to accept inputs from multiple trigger
sources simultaneously.

Pipelines are associated with exactly one trigger, which must now
be named in the configuration file.

Co-Authored-By: Monty Taylor <mordred@inaugust.com>

Change-Id: Ief2b31a7b8d85d30817f2747c1e2635f71ea24b9
diff --git a/TESTING.rst b/TESTING.rst
index 293bb83..56f2fbb 100644
--- a/TESTING.rst
+++ b/TESTING.rst
@@ -67,6 +67,19 @@
   .tox/py27/bin/activate
   testr failing --list
 
+Hanging Tests
+-------------
+
+The following will run each test in turn and print the name of the
+test as it is run::
+
+  . .tox/py27/bin/activate
+  testr run --subunit | subunit2pyunit
+
+You can compare the output of that to::
+
+  python -m testtools.run discover --list
+
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