afb-api-ws: Make weak ws-client by default (to be fixed)

The case of unicens service that can be here or not creates
problem to the binding 4a when the service is not here.
This commits makes the requirement of ws-client
socket optional. This unblocks the situation but need to
be fixed later.

Bug-AGL: SPEC-1151

Change-Id: I569fa781c003492a4c6a7ad84c5b140ecdb05b65
Signed-off-by: José Bollo <jose.bollo@iot.bzh>