CHEF-4086: Added resource_url_use_fqdn and s3_external_url configuration

By default resource_url_use_fqdn is false. When false, all URL
generation returned to the requestor will be based upon the value for
the Host header in the request.

This can be turned off by making resource_url_use_fqdn true.

Regardless of the resource_url_use_fqdn setting, if s3_external_url has
a non-nil value, then all URLs generated for uploading cookbook
artifacts will use that as the base url. The value for s3_external_url
should be a fully-qualified URL ("http://example.com")

This allows for the following configurations:

default - all generated urls based on Host: value. (Works across
proxying and load balancing)

fqdn - all generated urls based on the FQDN of the host.

vhost + s3_external_url - generate URLs based on Host: value except for
the S3 URLs.

fqdn + s3_external_url - generate URLs based on FQDN except for the S3
Urls.