Move charm archive logic to API server

The client no longer determines the charm archive URL
from the current API server address, but instead queries
the API server for a list of URLs to attempt. The uniter
will try each URL until one works.

Fixes https://bugs.launchpad.net/juju-core/+bug/1394976
Merge pull request #1212 from axw/lp1394976-api-uniter-charmarchiveurls

Move charm archive logic to API server

The client no longer determines the charm archive URL
from the current API server address, but instead queries
the API server for a list of URLs to attempt. The uniter
will try each URL until one works.

Fixes https://bugs.launchpad.net/juju-core/+bug/1394976

(Review request: http://reviews.vapour.ws/r/522/)
Merge master.