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.

I tend to err towards less debug messages when the code is debugged. This just gives us the temp directory name, which isn't really *that* useful, I think. I'm open to arguments to the contrary though.