Add read-replica support to the RDS module

[mtrbls]

There is a need to the RDS module to create Read Replica database.
A read replica is a read-only database maintained in sync with a source RDS.

Terraform supports this : replicate_source_db

Having one normal RDS and a read replica RDS would mean invoking the module twice, passing the db_id of the first one as the value to replicate_source_db field of the second one.

[digitalronin]

Why would we need to run the module twice? Couldn't you just define two RDS instances, one of which referred to the other, and have terraform figure out that there was a dependency?

[AntonyBishop]

Required for CLA migration from Template Deploy

[poornima-krishnasamy]

hashicorp/terraform-provider-aws#6749