Remove non-required `cloudflare_email`, fixes #84

[benvaljean]

• Only api_key actually requires email and the provider now fails if we include email but not api_key. - We never actually needed to submit our email to the provider as we are using an api_token.

Due to https://github.com/cloudflare/terraform-provider-cloudflare/issues/1807

Fixes https://github.com/hobby-kube/provisioning/issues/84

[pstadler]

Great PR, thank you!