Allow PPAs to be set NotAutomatic and ButAutomaticUpgrades

It would be nice if PPA owners were able to make their PPAs not all-or-nothing,
as they are currently.

That is, if we could allow owners to toggle a flag which sets NotAutomatic in
the Release files of the PPA. And, if NotAutomatic is (to be) set then allow
them also to choose to set ButAutomaticUpgrades too (this should probably
default to 'yes' though).

This would be useful for 'test' PPAs, and would mean that they can function
something like -backports does now.

I believe that these flags only work with apt in >= natty.