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.
this (``builddir[0]``) confuses me, isn't that taking the first chart of a string here? (I'm happy to be told I'm wrong)