Support requests specifying only one of min or max value per QoS parameter

QoSPolicy now uses a marked value (UNSPECIFIED_VALUE = -1) to indicate a value has not been specified.
Parsers for QoSPolicy now translate null values into UNSPECIFIED_VALUE, and vice-versa.

There is no need to change any driver implementation as QoSPolicy parameters are unused by now.

This patch fixes issue OPENNAAS-1367 (http://jira.i2cat.net:8080/browse/OPENNAAS-1367)