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.
Removes unused option and return value.
Merge pull request #28 from mrrrgn/by-builder-clobbers

Bug 1095615 - Adds an endpoint to clobber builddirs by buildername