MySQL Read replica implementation

[autero1]

This PR adds support for Cloud SQL read replicas.

Fixes #16

[autero1]

Yes, the outputs and related documentation are on my final todo list. The certificate outputs are something I don't use in the tests for the replicas, so I didn't finalize them. Wanted to push this out 90% complete to get regression feedback from the tests.

[autero1]

Until terraform 0.12 certain things are a dead end due to this: https://github.com/hashicorp/terraform/issues/17048

It's what was problematic with the IP addresses and now it's the same condition for server certs for the read replicas. If we're sure the count of a resource is never > 1, we don't have to use the splat operator. This is the case for the failover replica, so we can write:

data "template_file" "failover_certificate" {
  count    = "${var.enable_failover_replica}"
  template = "${google_sql_database_instance.failover_replica.0.server_ca_cert.0.cert}"
}

But now with potentially more than one read replica, we cannot do that. Using splat fails due to the terraform issue. The tests are currently failing because of this issue.

[autero1]

Until terraform 0.12 certain things are a dead end due to this: hashicorp/terraform#17048

I ended up

• Documenting this as a "Known Issue" in README
• Outputting the information JSON encoded, so it's both human and machine readable

I think this is a reasonably ok interim solution before we have terraform 0.12

[autero1]

No longer WIP