Add MinAdjustmentStep for EC2 Autoscaling

Applies #1562 from @ajdiaz, adding MinAdjustmentStep as an option for autoscaling policies when the adjustment type is `PercentChangeInCapacity` as well as PEP8 fixes and unit tests.

Added three new tests:

```bash
test_scaling_policy_with_min_adjustment_step (tests.unit.ec2.autoscale.test_group.TestCreateAutoScalePolicy) ... ok
test_scaling_policy_with_wrong_adjustment_type (tests.unit.ec2.autoscale.test_group.TestCreateAutoScalePolicy) ... ok
test_scaling_policy_without_min_adjustment_step (tests.unit.ec2.autoscale.test_group.TestCreateAutoScalePolicy) ... ok
```