Valid sub_roles for ASIC in chassis based systems with VoQ enabled

[m11dedhia]

Trying to understand if chassis based systems should have a third namespace type defined by the sub_role present in DEVICE_METADATA. This HLD clearly states all fabric ASICs in a chassis system environment are backend ASICs. Contrarily, this PR introduces a new concept of fabric_ns for each "Fabric" ASIC present on a VoQ system inside sonic-py-common/multi_asic.py. This is confusing and needs more clarity if the Fabric ASICs in a VoQ enabled supervisor of a chassis based system should have their sub_role as Fabric or BackEnd.

Current yang models for DEVICE_METADATA only mention two values available for sub_role