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.
Seems cleaner this way

No need to repeat the entire call for the two cases
it makes sense to skip the object wrangling and only handle the values
Merge pull request #6 from thekad/master

Add *instance* support for VPC and extra params