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.
if we're returning session, would be ideal to support session=None, none of the existing code cares about the return value.