pipeline: make the pipeline a singleton

In the future the pipeline will implement the table datastructures itself. We
don't want to replicate these read-mostly datastructures for each upcall thread,
so use a single pipeline instance per process.
pipeline: remove lookup function pointer

For now we are content to tie the pipeline implementation to OVSDriver.
remove pipeline utest

It wasnt't doing anything, and linking with OVSDriver is complicated.
pipeline: implement registration
factor out pipeline_standard into its own module
pipeline: integrate with Indigo pipeline interfaces
ivs: support setting pipeline name from the command line

The argument is optional, so the syntax is "--pipeline=standard-1.3".
pipeline: copy registered name
pipeline: disallow duplicate pipeline names
Merge into master from pull request #93:
Implement pluggable forwarding pipelines (https://github.com/floodlight/ivs/pull/93)