Implements SFC CRUD interface in OpenStack vimconn

Change vimconn_openstack.py to implement the SFC functions
specified by the SFC interface in vimconn.py.

The OpenStack VIM connector calls the networking-sfc Neutron
extension methods, whose resources are mapped to the VIM
connector's SFC resources as follows:
- Classification (OSM) -> Flow Classifier (Neutron)
- Service Function Instance (OSM) -> Port Pair (Neutron)
- Service Function (OSM) -> Port Pair Group (Neutron)
- Service Function Path (OSM) -> Port Chain (Neutron)

Also adds unit tests.

This relates to feature 638.

Change-Id: I501837bfaed6638a7b5f345fda547b746e4e7d45
Signed-off-by: Igor Duarte Cardoso <igor.duarte.cardoso@intel.com>