Previously the AMI creation accepted a static value for the AMI's block device's volume size.
This change allows the user to omit the `volume_size` attribute, in order to mimic the AWS API behavior, which will use the EBS Volume's size.
Also fixes a potential panic case when setting `iops` on the AMI's block device.
The `aws_ami` resource previously didn't have any acceptance tests, adds two acceptance tests and a full testing suite for the `aws_ami` resource, so further tests can be written, as well as expansion upon the other `aws_ami_*` acceptance tests
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAMI_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/09 20:18:22 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAMI_ -timeout 120m
=== RUN TestAccAWSAMI_basic
--- PASS: TestAccAWSAMI_basic (44.21s)
=== RUN TestAccAWSAMI_snapshotSize
--- PASS: TestAccAWSAMI_snapshotSize (45.08s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 89.320s
```
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