Remove routes/nat when disassociating FIPs

* Added provider router route removal when
  - deleting floating ips
  - disassociating floating ips
  - updating floating ips to remove the port association

* Added support for creating router's gateway in create_router

* update_floatingips now returns the updated fip dictionary

Fixes MN-1027
Signed-off-by: Joe Mills <joe@midokura.com>