Requests to the unified logical findit builder `findit_variable` in buildbucket currently require that the 'mastername' property be set to the buildbot master it would have, were it not a swarmbucket-based builder.

This is okay for the short term, as it avoids having multiple code-paths for supporting buildbot and swarming-based trybots. After the migration is done, however, we'll need a different way of letting the recipe know how to configure the builder, as the current method will be opaque and misleading.