This updates the branch cache (and associated connection mixin)
to include information about supported project merge modes. With
this, if a project on github has the "squash" merge mode disabled
and a Zuul user attempts to configure Zuul to use the "squash"
mode, then Zuul will report a configuration syntax error.
This change adds implementation support only to the github driver.
Other drivers may add support in the future.
For all other drivers, the branch cache mixin simply returns a value
indicating that all merge modes are supported, so there will be no
behavior change.
This is also the upgrade strategy: the branch cache uses a
defaultdict that reports all merge modes supported for any project
when it first loads the cache from ZK after an upgrade.
Change-Id: I3ed9a98dfc1ed63ac11025eb792c61c9a6414384
640059a67a