Add new resource vault_cert_auth_backend_config

[tmanninger]

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Relates OR Closes https://github.com/hashicorp/terraform-provider-vault/issues/1696

Release note for CHANGELOG:

- Add new resource "vault_cert_auth_backend_config"

Output from acceptance testing:

make testacc TESTARGS='-run=TestAccCertAuthBackendConfig_basic'
ok  	github.com/hashicorp/terraform-provider-vault/vault	3.869s
...