Add 'suggested' status type for parameters

This is a status for parameters which are not 'required' but are recommended
to be high value, if not always used, by downstream template users. This
commit does not add GUI-related changes to address this, as the interface
probably needs moving from checkbox- to dropdown-based status setting given
that there are now four, rather than three, mutually-exclusive statuses.

Change-Id: I104976e76d5ad6d586d9c818418ba6e16af53506
Merge "Add 'suggested' status type for parameters"