Driver-based fallback providers

As a deployer or a device vendor, I would like ONOS to better support use of multiple protocols for its fixed-function subsystems, so that I can broaden the support of existing devices and protocols.

Implementation Notes:

• Modify the fixed-function code subsystems to have a fallback provider for when there is no matching one for the specified DeviceId URI scheme
  Default*Provider, for FlowRule, Group, Meter, Packet
• These fallback providers would utilize the DriverService to determine whether the drivers for the subject device support the appropriate behaviour and if so, engage that behaviour
• A set of handler behaviours would be defined as an analog of their corresponding provider interfaces
  • PacketProvider would be backed by PacketEmitter
  • FlowRuleProvider would be backed by FlowRuleControl
• etc.
• The Default*Provider would then:
  • act as a proxy for the device driver subsystem
  • cache DriverHandler instances as necessary

See https://docs.google.com/a/onlab.us/presentation/d/1Gu3Nlb4YGPaOLqoT6OYB4MspYud8ZTQZwq3hKImGbAs/edit?usp=sharing