New fact "port_fragment"

The new port_fragment fact combines the existing forwarded_port fact
with a leading colon (:). This can be used in constructing
self-referential URLs. In labs-vagrant or if the settings[:http_port] is
80 (http default), the port_fragment fact will be undefined.

Change-Id: Ia438cc209a30bb132479fce2202e2c9f9dbd2c43