Timeout Support for builds/build scripts and default timeouts

Sometimes, due to developer errors, builds get stuck in loops that cause them to run forever

Currently launchpad itself does not offer many mechanisms to handle this

It would be good to have:
 - A way to specify a build timeout in build scripts and automatic build configs
 - A sane default timeout such as one or a few hours which the user can raise if needed

All builds that exceed the timeout would simply be cancelled by the system, with a notice that a timeout caused the build failure

Currently it seems that build logs get deleted when a build gets cancelled. This should not happen for system cancellations since it makes debugging a lot harder.