This allows for updates to size, type and iops
Fixes: #11931
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEBSVolume_update'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/15 22:35:43 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEBSVolume_update -timeout 120m
=== RUN TestAccAWSEBSVolume_updateSize
--- PASS: TestAccAWSEBSVolume_updateSize (53.57s)
=== RUN TestAccAWSEBSVolume_updateType
--- PASS: TestAccAWSEBSVolume_updateType (57.53s)
=== RUN TestAccAWSEBSVolume_updateIops
--- PASS: TestAccAWSEBSVolume_updateIops (53.63s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 164.753s
```
Paul Stack