apiserver/facade: add Registry.ListDetails method

[rogpeppe]

We add a ListDetails method to obtain information
on the available facades and also add AdminFacadeDetails in apiserver
for the Admin facade, which isn't available from apiserver.AllFacades.

This enables an external program to obtain information about
the juju API, for example to generate documentation
automatically or to do static compatibility checks.

Also export IsControllerFacade and IsModelFacade so that it's
possible to tell which facades are available in which mode.
In the future, it would be nice to be able to find which facades
are available to users as opposed to agents, but that's for
another PR.

Also remove some unused cruft from the tests (we only have
one admin facade version at the moment).

QA:

go get github.com/rogpeppe/misc/cmd/jujuapidoc github.com/rogpeppe/misc/cmd/jujuapidochtml
jujuapidoc > /tmp/apidoc.json
jujuapidochtml /tmp/apidoc.json > /tmp/apidoc.html
open /tmp/apidoc.html
# observe the beauty of the Juju API.

[rogpeppe]

This is the only field removed.

[rogpeppe]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju