[Bloom] Disable master branch build and limit to Xenial
[Bloom] Disable master branch build and limit to Xenial

Bloom packaging currently seems to have problems when running on
Bionic, so we limit the packaging agents to the Xenial ones.
Also, the option to trigger a default branch build after a non-default
build is confusing and it's not clear when we would need that, so
this change disables this option.

Change-Id: I34eea454656627d6e12ade6f2838168fa2a75ca3

sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts