Add support for host domains
Add support for "host domains" - groups of hosts exclusive to a particular run of tests in the database ("Scene" within a "Scenario").
Allow domains to have their own "host_requires", narrowing down the hosts they are targeting. Allow a host type to belong to zero, one, or more domains. Make domains be aware of any forced hostnames their host types are using, considering them parts of the domain.
When instantiating a host for a domain, in addition to normal host_requires, also add host_requires for the domain, as well as negated host_requires for all other domains, plus exclude any forced hostnames from the other domains.
Allow command-line tools to limit domain selection.