MinimumBandwidthRule direction incorrectly defined

In neutron-qos.yang minimumbandwidth-rule-attributes has a direction of type constants:direction-minimum-bandwidth-rule. However, that type is a "terminal node" with no inheriting directions. In addition, since direction appears in a few places there is no reason to not use the generic:

identity direction-base

{     description "Base identity for direction"; }

identity direction-ingress

{     description "Ingress direction.";     base direction-base; }

identity direction-egress

{     description "Egress direction.";     base direction-base; }

This change will require coordination with consumers of the existing yang model, e.g., netvirt and networking-odl.

h/t to Vishal Thapar for finding this