charmload: some refactoring to address review comments

We add the capability for charmload to take parameters from the same
config file that charmd uses, splitting the config reading code into a separate
package to enable that.

Various other changes as suggested in review.

Some tests will arrive in the next PR - for now, I've done
some verification on the command line.

Arbitrary change. I tend to dislike arbitrary changes because when looking through a changelog one may get the impression that there was a reason for it, unless I'm missing something and there is a good reason to use the branch dir, in which case I like to see that mentioned in the commit message.