[CHEF-4176] update `knife configure -i` for Chef 11

* Chef 11 defaults to HTTPS on 443
* Chef 11 admin key is in /etc/chef-server/admin.pem
* Chef 11 validator key is in /etc/chef-server/chef-validator.pem

Ref: http://www.opscode.com/blog/2013/03/11/chef-11-server-up-and-running/
[CHEF-4176] reduce duplication in knife configure tests

Tests contain repeated references to the default admin key path,
validator path, and chef_server_url. Replace literals with let bindings
to avoid duplication. This is especially desirable for the negative
tests (e.g. `should_not match`) where drift in the values could render
the tests worthless.