Support passing ppa arguments into the image build as well

[vorlonofportland]

There are occasions when you want to support specifying a ppa to configure in the target image, not just in the build chroot. This is a first pass at implementing this.

In Launchpad itself, these two concepts are kept separate. The one is specified in terms of the archive that the image is built "in"; the other is EXTRA_PPAS that are passed into the buildlivefs command. So by trying to use the single --ppa option for both we are conflating a bit. Do we care?

Further limitations:

• If the ppa is a private ppa, do not pass it into the build. This isn't supported by launchpad-buildd today.
• You may want more than one ppa. For instance, we have some custom images that use one (private) ppa for the source of the build scripts, but want to configure a separate (public) in the target image. I haven't implemented this yet as part of this branch, because it takes some thinking how we want to match up when we have a mix of --ppa and --ppa-key options on the commandline.