Added the Nexus driver

1. This is not tested, because there is no Nexus switch in the haas environment 
2. The Nexus driver possibly works only with one switch. This would be the case if the Nexus switch does not support switch stacking.
We should probably get rid of the redundant code here too. From there,
it would be nice to test this.

Quoting Ryan Abouzahra (2014-09-05 16:52:38)
>    In haas/drivers/nexus.py:
> > +        sys.Exit(1)
> > +
> > +
> > +
> > +def get_new_network_id(db):
> > +    vlan = db.query(Nexus_Vlan).filter_by(available = True).first()
> > +    if not vlan:
> > +        return None
> > +    vlan.available = False
> > +    returnee = str(vlan.vlan_no)
> > +    return returnee
> > +
> > +def free_network_id(db, net_id):
> > +    vlan = db.query(Nexus_Vlan).filter_by(vlan_no = net_id).first()
> > +    if not vlan:
> > +        pass
> 
>    Now that Andy and I have abstracted a lot of the code out of the dell
>    driver, what's the next step in regards to this pull request?
> 
>    --
>    Reply to this email directly or [1]view it on GitHub.
> 
> References
> 
>    1. https://github.com/CCI-MOC/haas/pull/225/files#r17197478