Fix aws_db_instance to not recreate each time
Fix aws_db_instance to not recreate each time

Several of the arguments were optional, and if omitted, they are
calculated. Mark them as such in the schema to avoid triggering an
update.

Go back to storing the password in the state file. Without doing so,
there's no way for Terraform to know the password has changed. It should
be hashed, but then interpolating the password yields a hash instead of
the password.

Make the `name` parameter optional. It's not required in any engine, and
in some (MS SQL Server) it's not allowed at all.

Drop the `skip_final_snapshot` argument. If `final_snapshot_identifier`
isn't specified, then don't make a final snapshot. As things were, it
was possible to create a resource with neither of these arguments
specified which would later fail when it was to be deleted since the RDS
API requires exactly one of the two.

Resolves issue #689.

terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws