bootstrap clusters should get default settings

[jpeach]

When we build a bootstrap cluster in internal/envoy/bootstrap.go, they have hard-coded Cluster settings that can go out of sync with the settings we use for service clusters. There's an argument that these have a different traffic profile and so they should be different, but a lot of the settings (e.g. lower the default buffer size) make sense here too.

[jpeach]

Ref #1375