Allow for string peer parameter

[gbregman]

In control/grpc.py code we have:

peer = context.peer().split(":", 1)

In some case we simulate the GRPC call ourselves and pass an empty string context instead of the actual object. In these cases we get the error:

ERROR grpc.py:851 (1): Got exception trying to get peer's address
Traceback (most recent call last):
  File "/src/control/grpc.py", line 838, in get_peer_message
    peer = context.peer().split(":", 1)
AttributeError: 'str' object has no attribute 'peer'

We need to check for this case and not display an error.