Refactor InstanceBroker.StartInstance to return result struct

StartInstance will soon need to grow another result for
reporting back block devices allocated to a provisioned
instance. Rather than having to touch every bit of code
each time we expand the set, use a result struct.
Merge pull request #1100 from axw/environs-startinstanceresult

Refactor InstanceBroker.StartInstance to return result struct

StartInstance will soon need to grow another result for
reporting back block devices allocated to a provisioned
instance. Rather than having to touch every bit of code
each time we expand the set, use a result struct.