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.

We make the temp file for the archive inside the existing temp directory
for a slightly simplification of the code.

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.

This is not relevent to this change, but I'm not sure whether this is the best thing to do here. why bother to check the mongoDB connection on startup? It could get dropped at any time so we have to cope with that anyway. does it really matter if we can't connect at startup?