Add Toggle for Custom RDS ParameterGroup

[ferricoxide]

Problem Description:

Custom RDS ParameterGroups can prevent doing in-place upgrades of PGSQL RDS DBs. Appears upgrades work when using default ParameterGroups (at least when executed outside the CFn context). Need to add a toggle for flipping back and forth between custom and generic when doing stack-updates

Expected Behavior:

Upgrading from 9.x to 10.x succeeds even if custom ParameterGroups are attached.

Actual Behavior:

Upgrading from 9.x to 10.x may fail if custom ParameterGroups are attached.

Fix recommendation:

Add a toggle to RDS template to flip between custom and default ParameterGroup for use during CFn-managed DB version upgrades.