fix(mwan): fixing validation on default rule

[Tbaile]

Fixing issue when editing default rule, the field was wrongly addressed as required even if when editing it's a mere placeholder.

Ref:

• Internal discussion: https://mattermost.nethesis.it/nethesis/pl/znaqcfmdujfh3kknuq755s5pse

[andre8244]

If we ever need to modify again useRuleForm or usePolicyForm , I suggest renaming them, as it's difficult to recognize their connection to multiwan.