Description of problem:
It is not possible to add a brick as arbiter to an existing volume in replica setup.

Version-Release number of selected component (if applicable):
3.7.8+

How reproducible:
always

Steps to Reproduce:
1. create replica 2 volume
2. deploy another node to extend the volume
3. there is no way to add the new node as 3rd brick and to act as an arbiter of the new setup

Actual results:
One has to destroy the volume and create a new one in the desired setup with arbiter. One is only able to add the new brick as another full replica node.

Expected results:
the `add-brick` command should accept the arbiter option

Additional info: