DomainControllerImpl treats some ops intended for a single host as domain-wide

See comments on AS7-270.

If DomainControllerImpl analyzes an operation and determines it requires a two-phase approach (exec on HostController(s) and then push to servers) the master DC treats it as a domain-wide op even if it's entirely limited to a single remote host. If it's limited to a single remote host, the master DC should route it to that host and let it coordinate the two-phase op.