Adds a validation function for cloudflare record types. Also adds an apply-time validation for the record's content based on record type. Currently only validating `A` and `AAAA` records, can be expanded to verify record content for every possible record type in the future.
```
$ make test TEST=./builtin/providers/cloudflare
==> 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
==> Installing errcheck...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/13 14:41:37 Generated command/internal_plugin_list.go
TF_ACC= go test ./builtin/providers/cloudflare -timeout=30s -parallel=4
ok github.com/hashicorp/terraform/builtin/providers/cloudflare 0.018s
```
Fixes: #11173
Jake Champlin