Fix restoring default instance specs in group policies

"default" was not accepted as a valid input value for instance specs in
group policies, due to a bug introduced in 2cc673a3 (and released with
2.6.0). Added QA for this and another similar case.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>