provider/aws: Validate WAF metric names

[tomelliff]

WAF metric names must be alphanumeric only and it would be useful to have this validated during a plan rather than fail on apply.

This bit us recently when I merged some code to master that used underscores for the metric name when the plan looked fine. Not a huge issue but should have been caught on the plan.

See http://docs.aws.amazon.com/waf/latest/developerguide/web-acl-rules.html#web-acl-rules-creating and
http://docs.aws.amazon.com/waf/latest/developerguide/web-acl-working-with.html#web-acl-creating

[tombuildsstuff]

Hi @tomelliff

Thanks for this PR - it LGTM :)

The tests pass:

$ make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestValidateWafMetricName'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/24 10:55:07 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestValidateWafMetricName -timeout 120m
=== RUN   TestValidateWafMetricName
--- PASS: TestValidateWafMetricName (0.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	0.019s

Thanks!

[ghost]

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