add parent object to Service Group class

[bvilajol]

Environment

• Nautobot version:
• nautobot-plugin-firewall-model version:

When you define a tenant group, you can select another object as parent. That leads to the ability to anidate this object.

Service Group would need same approach. This way amount of objects decreases as well as different levels of anidation of Services could be implemented.

Proposed Functionality

SERVICE1 SERVICE2 SERVICE3 SERVICE4 SERVICE5 SERVICE6

GROUP1 = SERVICE1+SERVICE2 GROUP2 = SERVICE3+SERVICE4 GROUP3= SERVICE4+SERVICE5

GROUP4, parent from GROUP3 + SERVICE4