log output should clearly print out step /instances

[pjenvey]

Loads should tell us more information about a plan's steps: especially the name of the plan and its allocated instances.

When allocating new instances you can discern what instances belong to which step:

[2017-02-28 15:35:49,439][18278] Allocated instances: [Instance:i-0e4c005cd189cfe78]
[2017-02-28 15:35:49,524][18278] Allocated instances: [Instance:i-07666c0cfd5332709]

(referencing Step 1, then Step 2)

but this could be clearer, and you don't get any of this information when it recovers instances.