Add *instance* support for VPC and extra params

* Lower the amount of DB queries done (fetch everything once, then
  process it in memory
* If you set an argument `is_vpc` clusto will instead look for the
  security group IDs, since boto needs a different parameter to
  run_instances when using vpc (I think that's... pretty dumb, ticket:
  https://code.google.com/p/boto/issues/detail?id=521 )
* You can add whatever extra `ec2_` parameters anywhere and they will be
  passed to run_instances() verbatim. Such is the case as
  `ec2_subnet_id` being passed as `subnet_id` and being able to boot up
  instances in VPC

Keep in mind that full support for VPC is still not done, i.e. VPC
specific objects and the like.