Bug 1095615 - Adds an endpoint to clobber builddirs by buildername

Existing services, like buildapi's selfserve-agent rely on the abilty to
clobber all of the builddirs associated with a buildername in bulk. This
endpoint was added with such services in mind.
nit: no users, or tests for commit=True (either at least test it, or omit it, imo)