Update controller multiplexer to handle the new Build Environment table

[nicholasdavidson]

As per #114 if we extend the schema and the client controller will need updating so we can select from suite_arches, left join on to build env and multiplex where appropriate.

We'll also need to update buildrequest or one of it's members in model to carry the build environment to the client.

[SiHaswell]

Controller now multiplexing jobs from svn hook. Also handling build_env for job retry, superceded job cancel.