Git driver

This patch improves the existing git driver by adding
a refs watcher thread. This refs watcher looks at
refs added, deleted, updated and trigger a ref-updated
event.

When a refs is updated and that the related commits
from oldrev to newrev include a change on .zuul.yaml/zuul.yaml
or zuul.d/*.yaml then tenants including that ref is reconfigured.

Furthermore the patch includes a triggering model. Events are
sent to the scheduler so jobs can be attached to a pipeline for
running jobs.

Change-Id: I529660cb20d011f36814abe64f837945dd3f1f33
diff --git a/tests/fixtures/zuul-git-driver.conf b/tests/fixtures/zuul-git-driver.conf
index b24b0a1..23a2a62 100644
--- a/tests/fixtures/zuul-git-driver.conf
+++ b/tests/fixtures/zuul-git-driver.conf
@@ -21,6 +21,7 @@
 [connection git]
 driver=git
 baseurl=""
+poll_delay=0.1
 
 [connection outgoing_smtp]
 driver=smtp