Implement pluggable forwarding pipelines

Reviewer: @poolakiran

The Indigo forwarding pipeline configuration interfaces have existed for a while, but so far they've been no-ops in IVS. This pull request implements a registration facility and initializes/deinitializes the pipeline implementations when the controller commands it.

The initial use will be to let the controller pick the OpenFlow forwarding model (1.0 or 1.3). Right now this is configured at startup with the --openflow-version command line option.