Switch net-config templates to use OS::stack_id

Switch net-config templates to use OS::stack_id

This patch removes the custom config_id outputs and replaces
it with OS::stack_id which allows us to just call get_resource
in the parent stack.

The motivation for this change is we'll be adding more os-net-config
templates and it would be nice to take advantage of this newer
template feature.

Change-Id: I6fcb26024b94420779b86766e16d8a24210c4f8e