Use 'seed' roles to guarantee we don't init cluster singleton too early

Hey there,
Here's one of the suggestions that will help a little to make the singleton more resilient.
I also opened an issue https://www.assembla.com/spaces/akka/simple_planner#/ticket:3986 so we can keep track of changes around this. For now I'll update the docs with some patterns.

This relates to: https://groups.google.com/forum/#!topic/akka-user/ns7DPHGYbIk (where I'm currently finishing a big write up why this happens and what we can do about it).

Thanks for bringing this up, 
happy hakking!