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.
Really, we ought to be abstracting out the common bits that are going to
apply to any vlan-based switch driver -- which is most of the code.

Quoting Ryan Abouzahra (2014-09-03 11:29:57)
>    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
> 
>    I'd just like to note that pull request [1]#230 fixes a bug here for
>    the Dell driver and is pending review. It seems logical to me that
>    [2]#230 should be merged first, then this can be fixed and merged.
>    [3]@gsilvis [4]@zenhack?
> 
>    --
>    Reply to this email directly or [5]view it on GitHub.
> 
> References
> 
>    1. https://github.com/CCI-MOC/haas/pull/230
>    2. https://github.com/CCI-MOC/haas/pull/230
>    3. https://github.com/gsilvis
>    4. https://github.com/zenhack
>    5. https://github.com/CCI-MOC/haas/pull/225/files#r17057362