Fixed dhcp & gateway ip conflict in PLUMgrid plugin

   * Adjust dhcp IP if conflicts with gateway IP
   * Added unit test case

Closes-Bug:1333442
Change-Id: Iaa4e63faf28b783e6b5dbc50035ca50ccde9951a
(cherry picked from commit cba1493de269937f7df761fbacbc27d6e0fa6fa3)
Merge "Fixed dhcp & gateway ip conflict in PLUMgrid plugin" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable