Launchpad integration for sbuild

 This includes two sbuild hooks and a command line tool to setup and
 maintain build chroots that are as close as possible to a standard
 Launchpad sbuild chroot.
 .
 The sbuild-launchpad-chroot command can be used to setup, update and
 destroy those chroots. A schroot hook will also automatically update
 them should an update be available at the start of a build.
 .
 Those chroots also come with a series of useful aliases allowing the
 user to easily choose which pockets and components should be available
 for the build.