Fix use of EXTERNAL_URL in rpm installs

RPM installs use a posttrans script to call the upgrade.rb script. We don't pass EXTERNAL_URL there, which breaks functionality. If we need to have posttrans script, we should have the external_url identification logic there also.

Passing variables/state between maintainer scripts is not an easy task, as it involves storing a file in one script and reading it again in another script. Hence, duplicating the necessary content from postinst script is the easier.