Add support for distributed routers

[Fodoj]

Even though it is not referenced in API ref, it is official feature, available in neutron cli, for example: http://docs.openstack.org/cli-reference/neutron.html

[jtopjian]

If you run neutron --debug router-create with the --distributed flag, are you able to see the "distributed" info in the request and response? If so, I think it would be helpful to update the JSON fixtures in the requests_tests.go file.

[Fodoj]

@jtopjian added, thanks

[jrperritt]

+2