Revert "discovery: import nom"

This reverts commit 9c760ca3d1be8662bfd52049af31d529cf8fe114.
Revert "openflow.discovery: Add Link objects to NOM once discovered. This will be noved to an adapter later. Same for Hosts(hosttracker) and Switches(openflow.topology)"

This reverts commit 525109d6f76cf99615a367a8c26d4264bbc817cc.

Conflicts:

	pox/openflow/discovery.py
Revert "l2_learning and discovery: install flow_mods through NOM"

This reverts commit 9460b7f144c7bb79e21c1bfcba5705b0762dcc4f.

Conflicts:

	pox/forwarding/l2_learning.py
	pox/openflow/discovery.py