Charm Store policy updates and refinement for 2.0

[Jorge O. Castro]

Thanks for the feedback Stuart, I've pushed up a new revision.

> I think the acceptable software sources needs to be expanded.

I've added your recommendations for this section except for:

> In addition, any software sources not in the main Ubuntu or CentOS
> archives should be listed in configuration items that can be
> overridden rather than hard coded in the charm

I've changed this to a MUST as it's not that much work to do this and
the effort seems trivial compared to forcing users to mangle a charm
just to get it to deploy on production systems without egress.