Add PWS Login Server link placeholders.

- To facilitate overriding them in cf-aws-stub (cloudfoundry/deployments-aws) for testing on A1.

This completes a pull request from @rmorgan on deployments-aws:
@see https://github.com/cloudfoundry/deployments-aws/pull/7

https://www.pivotaltracker.com/story/show/70697714

Thanks,
Rob Gallagher