* provider/aws: Refresh `aws_autoscaling_policy` from state on 404
Fixes#8386
When an Autoscaling Group Or an Autoscaling Group Policy has been
deleted manually, terraform was throwing an error as follows:
```
* aws_autoscaling_policy.increase: Error retrieving scaling policies: ValidationError: Group sandbox-logs-logstash-wxhsckky3ndpzd7b3kmyontngy not found
status code: 400, request id: 56a89814-6884-11e6-b3a8-d364cf04223b
```
We now refresh from state on a ValidationError - this is a common 4xx error according to AWS documentation http://docs.aws.amazon.com/AutoScaling/latest/APIReference/CommonErrors.html
```
%make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAutoscalingPolicy_disappears'
==> Checking that code complies with gofmt requirements...
/Users/stacko/Code/go/bin/stringer
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAutoscalingPolicy_disappears -timeout 120m
=== RUN TestAccAWSAutoscalingPolicy_disappears
--- PASS: TestAccAWSAutoscalingPolicy_disappears (203.61s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 203.633s
```
* Update resource_aws_autoscaling_policy.go
Unlike SimpleScaling policies, StepScaling policies require one or more
"steps", which are interval ranges in which a tracked metric can lie.
Policies can then execute scaling adjustments wedded to these steps.
This commit also adds a slew of additional policy attributes which are
only applicable to step policies.
Paul Stack