Improved waiting for stabilisation in Auto-Scaling deploys

We had a failing deploy on the Membership team:

https://riffraff.gutools.co.uk/deployment/view/582120b7-0d4e-4759-8bdd-21d334b98aae

These are the relevant lines:

```
[11:28:04] task DoubleSize Double the size of the auto-scaling group in Stage(PROD) for apps App(frontend)
[11:28:04] task WaitForStabilization Check the desired number of hosts in both the ASG and ELB are up and that the number of hosts match
[11:28:05] task HealthcheckGrace Wait extra 0ms to let Load Balancer report correctly
[11:28:05] task WaitForStabilization Check the desired number of hosts in both the ASG and ELB are up and that the number of hosts match
[11:28:05] task CullInstancesWithTerminationTag Terminate instances with the termination tag for this deploy
[11:28:08] Unhandled exception in task CullInstancesWithTerminationTag Terminate instances with the termination tag for this deploy
[11:28:08] Unhandled exception in deploy for Membership::frontend (build 3589) to stage PROD
```

You can see that the 1st and 2nd WaitForStabilizations executed almost
instantly (and we haven't had a HealthcheckGrace duration configured on our
team, thus the intervening HealthcheckGrace executed instantly too). So,
all these checks executed before the AWS API could catch up and report the
updated values from the DoubleSize step. Consequently the deploy proceeded
with only 3 instances in our ASG & ELB, not the desired double-capacity of
6, and then the instance-culling failed.

Setting a HealthcheckGrace would alleviate our problem - but then the
first WaitForStabilization becomes completely pointless. So this commit
removes the 1st WaitForStabilization - and sets the default
HealthcheckGrace to 3 seconds, which hopefully is both harmless & helpful!
Use a more reasonable default grace period...

As @gklopper says, if you can provision an AWS box under 20 seconds, you
deserve a medal:

https://github.com/guardian/deploy/pull/219#issuecomment-68044786
Merge pull request #219 from guardian/improve-waiting-for-asg-stabilisation

Improved waiting for stabilisation in Auto-Scaling deploys