Bp/neutron ipam

1) Removed used stick method _is_ip_avaliable()
2) Remove unused arguments in _allocate_fixed_ips(), and updated calling code
3) Renamed method ipam() to get_ipam_driver()

This part looks strange, since we call it from NeutronIPAMPlugin:
>   def get_ipam_driver(self):
>        return manager.NeutronManager.get_ipam()
And my expectation, that cls is not a NeutronIPAMPlugin here.