Support scalable storage settings separately for read-replica vs. primary

[dweinshenker]

What

• Enables read-replicas to set their own value for storage_size_mib to specify disk space
• Resolves some outdated Mongo tests using older versions
• Fixes resize issue for clusters when omitting storage_size_mib

[dweinshenker]

Need to add some wait between a new test, there is an issue during resize of a replica

=== CONT  TestAccDigitalOceanDatabaseReplica_Resize
    resource_database_replica_test.go:120: Step 4/4 error: Error running apply: exit status 1

        Error: Error resizing database replica: PUT https://api.digitalocean.com/v2/databases/a064eaf3-6c3c-43c8-acb3-0896c539bd0e/resize: 412 (request "917dc244-069c-4a54-8c33-2396ae5ba091") update cluster cannot be performed. Reason: cluster is undergoing maintenance

          with digitalocean_database_replica.read-01,
          on terraform_plugin_test.tf line 11, in resource "digitalocean_database_replica" "read-01":
          11: resource "digitalocean_database_replica" "read-01" {

--- FAIL: TestAccDigitalOceanDatabaseReplica_Resize (822.91s)

[danaelhe]

Looks good 👍
Thank you!