Add resource_url helper and expose base_resource_url
Add resource_url helper and expose base_resource_url

This exposes the base_resource_url in the platform config, and adds a
resource_url helper function to assist in constructing URLs.

Signed-off-by: Mark Anderson <f1b5a91d4d6ad523f2610114591c007e75d15084@chef.io>

chef/chef-server,juliandunn/chef-server-1,juliandunn/chef-server-1,juliandunn/chef-server-1,juliandunn/chef-server-1,chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server,juliandunn/chef-server-1,chef/chef-server