Support for Dummy slaves and/or Builders that don't require slaves

[catlee]

Sometimes it's useful to have builders that have nothing but Dummy steps, or other steps that don't require a slave.

In these cases it doesn't make sense to require a connected build slave to run these builders.

Ideally 'checkconfig' would error out if you have a builder that has steps that require a real slave but don't have any configured.

[dustin]

This is also useful for Trigger-only builds that just trigger a combination of other builds, or for builds that only occur on the master (e.g., a nightly build to clean up old artifacts)

[dustin]

I think that this should end up being implemented as a build that just doesn't require a slave, and the build request distributor should handle that appropriately.