Add description field in floatingip port forwardings API

Problem Description
===================

As users create and update theirs floating ip rules, the reason
behind those rules might get lost throughout time. Moreover, in
an environment with many people writing rules, it is important
to track down the reason behind each one of the rules
created/added in a floating IP port forwarding configuration.
The addition of a description field would allow operators to
determine the reason why a rule was created and help the users
to know if the existence of a rule is still reasonable.

Proposed Change
===============

To address the described scenario, we propose to create a new
“description” field in the Neutron’s Floating IP port forwarding
rules API JSON. This new field will be a nullable String
containing the description/reason why this new port forwarding
rule is being created.

Example of a modified JSON:

{
   "port_forwarding":{
      "protocol":"tcp",
      "internal_ip_address":"172.16.0.7",
      "internal_port":2230,
      "internal_port_id":"b67a7746-dc69-45b4-9b84-bb229fe198a0",
      "external_port":2230,
      "description":"Some description"
   }
}