Removed unsed static method _is_ip_avaliable()
Removed used argugments in _allocate_fixed_ips(), and updated calling
code accordingly.
Renamed method ipam() to get_ipam_driver()
Fixed bug introduced in the renaming of NeutronIPAMPlugin.ipam() method.
Merge pull request #2 from yuewko/bp/neutron-ipam

Remove some unused stuff and rename ipam() to get_ipam_driver()