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.

Unfortunately that conflicts somewhat with having -u override the user/password from the config file (you can't tell if -u has been specified). We *could* still apply a default, but since no-one should be using the default user name and password anyway and when testing it's easier just to name the same config file that charmd was started from, I don't think it matters much.