Add support for 'ignore-notify' in git SCM

This option is named "Don't trigger a build on commit notifications"
in the Jenkins Web UI.

Change-Id: I338af61f8bfdef1545d1ea206e6874cda38957cd

tests/scm/fixtures/git-ignore-notify02.yaml

@@ -0,0 +1,7 @@
+scm:
+  - git:
+      url: https://github.com/openstack-infra/jenkins-job-builder.git
+      branches:
+        - master
+      clean: true
+      ignore-notify: false