Improved validation mechanism for QoS rules with port types

Currently neutron API can return what rules are supported, in the context of ML2: by returning the minimum subset of rules supported by all mechanism drivers.

Mechanism drivers in ML2 plugin can specify a list of QoS rule types which are supported. For example openvswitch mech_driver will report "bandwidth_limit_rule" (which is only for egress traffic in fact) and "dscp_marking" rule.

There are two problems with such simple check:

 - if rules are created or policy is applied to port there is no validation at all. It means that for example policy with dscp_marking rule can be applied to port bound with linuxbridge or sriov mech_drivers without any errors.

 - there is no possibility that mechanism driver will support some rules only with specific parameters. For example bandwidth_limit_rule could have parameter "direction" with values "ingress" and "egress" and some of mechanism drivers could only supports bandwidth limit with direction=egress.

We propose changes to how supported rule types are calculated and how validation works.