This sets a reasonably secure TLS configuration for the GRPC client that
federator uses to connect to other federators (or rather, nginx ingresses,
assuming a wire-server instance on the other side).

## Summary of changes

 - Enabled TLS validation and specifying a CA when creating a GRPC client
 - Added new configuration options for federator and corresponding docs
   (`docs/reference/config-options.md`)
 - Added integration test for the GRPC client against a local nginx playing the
   role of ingress
 - Changed end2end tests so that certificates for the ingress are created on
   the fly (using correct domains)
 - Added unit tests for the GRPC client using a mock http2/TLS server