Add MinAdjustmentType for autoscaling.

When policy_type is PercentChangeInCapacity, allows values of
min_adjustment_type, which set the AWS MinAdjustmentType value for
autoscaling policies.
PEP8 fixes for EC2 MinAdjustmentStep
Add unit tests for EC2 auto scaling policy MinAdjustmentStep
Only include MinAdjustmentStep as a parameter if it has been set in order to prevent changing existing behavior; updated unit tests to show this behavior.
Merge remote-tracking branch 'taylordt/ec2-min-adjustment-type' into develop