Bug 1022763: configure mockbuild via runner; r=simone

This configures mockbuild based on the location of the node, but in such
a way that puppet is not required.  Instead the configuration is based
on the placement/availability-zone instance metadata, and if that is
empty (e.g., in-house), then uses the public vhost,
http://mockbuild-repos.pub.build.mozilla.org.