provider/aws: Validate regular expression passed via the name_regex attribute.

[kwilczynski]

This commit adds a simple ValidateFunc to check whether the regular
expression that was passed down via the name_regex attribute is valid.

[kwilczynski]

The unit test is passing:

$ make test TEST=./builtin/providers/aws TESTARGS='-v -run=TestResourceValidateNameRegex'
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/10/26 12:02:35 Generated command/internal_plugin_list.go
TF_ACC= go test ./builtin/providers/aws -v -run=TestResourceValidateNameRegex -timeout=30s -parallel=4
=== RUN   TestResourceValidateNameRegex
--- PASS: TestResourceValidateNameRegex (0.00s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    0.017s

[kwilczynski]

Resolves #9608.

[stack72]

LGTM! Thanks for fixing this so fast @kwilczynski :)

[ghost]

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