fix(reserved instances): use var.instance_type to determine RI offering instead of optional db_instance_class var #141

Job	Run time
terraform-module / CI / Format HCL	19s
terraform-module / CI / bats	32s
terraform-module / CI / Find modules	3s
terraform-module / Readme	0s
terraform-module / CI / Codeowners validate	11s
terraform-module / CI / Labels Validate	1s
terraform-module / CI / Lint (.)	12s
terraform-module / CI / Lint (./examples/basic)	22s
terraform-module / CI / Lint (./examples/complete)	14s
terraform-module / CI / Lint (./examples/enhanced_monitoring)	19s
terraform-module / CI / Lint (./examples/postgres)	15s
terraform-module / CI / Lint (./examples/serverless_mysql)	13s
terraform-module / CI / Lint (./examples/serverless_mysql57)	14s
terraform-module / CI / Lint (./examples/serverlessv2_postgres)	13s
terraform-module / CI / Lint (./examples/with_cluster_parameters)	11s
terraform-module / ci	3s
terraform-module / Release	0s
	3m 22s

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