This supplies a per-project default value for Zuul's default-branch
based on what the default branch is set to in GitHub. This means
that if users omit the default-branch setting on a Zuul project
stanza, Zuul will automatically use the correct value.
If the value in GitHub is changed, an event is emitted which allows
us to automatically reconfigure the tenant.
This could be expanded to other drivers that support an indication
of which branch is default.
Change-Id: I660376ecb3f382785d3bf96459384cfafef200c9
57a9c13197