[AWS|ASG] parse ScalingPolicy MinAdjustmentStep

For some reason this attribute was missing from the parser. Proof that
it exists:

http://docs.aws.amazon.com/AutoScaling/latest/APIReference/API_ScalingPolicy.html
Merge pull request #2001 from bgentry/asg-policy-min-adjustment

[AWS|ASG] parse ScalingPolicy MinAdjustmentStep