provider/aws: Allow updating constraints in WAF SizeConstraintSet + no constraints

[radeksimko]

This is to fix a bug very similar to #10403 and #11959

TL;DR We were not deleting any constraints except when deleting the whole Set.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSWafSizeConstraintSet_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 09:23:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSWafSizeConstraintSet_ -timeout 120m
=== RUN   TestAccAWSWafSizeConstraintSet_basic
--- PASS: TestAccAWSWafSizeConstraintSet_basic (39.27s)
=== RUN   TestAccAWSWafSizeConstraintSet_changeNameForceNew
--- PASS: TestAccAWSWafSizeConstraintSet_changeNameForceNew (462.83s)
=== RUN   TestAccAWSWafSizeConstraintSet_disappears
--- PASS: TestAccAWSWafSizeConstraintSet_disappears (30.20s)
=== RUN   TestAccAWSWafSizeConstraintSet_changeConstraints
--- PASS: TestAccAWSWafSizeConstraintSet_changeConstraints (46.09s)
=== RUN   TestAccAWSWafSizeConstraintSet_noConstraints
--- PASS: TestAccAWSWafSizeConstraintSet_noConstraints (25.06s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	603.492s

cc @yusukegoto

[grubernaut]

LGTM

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.