Fix encoding/decoding of /capable-switch/resources/queue/port/ element

This element has been represented by an integer but the specification
says that it should reference /capable-switch/resources/port/resource-id,
which is a string.

(Motivated by FlowForwarding/LINC-Switch#302).
Merge pull request #4 from FlowForwarding/inexistent-queue-of-config

Fix encoding/decoding of /capable-switch/resources/queue/port/ element