I7b4108772e8370e8f51971caf40ecd23e9f977e9 has been merged but missing
converter for firewall_rule. This commit adds a new converter to modify
string and applies this one for following attributes both fwaas and
fwaas_v2.
* source_port as in firewall_rule
* destination_port as in firewall_rule
Change-Id: Iced5a588b2618735cbe6fc081e42e49896d7a91a