add environment variable to disable kflow installation

When debugging the controller (and using verbose logging on the switch)
it's often helpful to see the pipeline logs for every packet. This flag
prevents IVS from installing kernel flows, so every packet will be
received as an upcall.

Set it with the environment variable IVS_DISABLE_KFLOWS=1.
Merge into master from pull request #141:
add environment variable to disable kflow installation (https://github.com/floodlight/ivs/pull/141)