Give scan callers a context for tracking local interface

When trying to leverage autodetect strategy for net config,
will need to keep track of receiving interface.  For now
only use ipv6 for this capability, as getting the incoming nic
for ipv4 would be more work.

confluent_osdeploy/common/opt/confluent/bin/apiclient

@@ -111,6 +111,8 @@ def scan_confluents():
                     current['isdefault'] = True
                 elif line.startswith(b'MGTIFACE: '):
                     current['mgtiface'] = line.replace(b'MGTIFACE: ', b'').strip().decode('utf8')
+            if len(peer) > 2:
+                current['myidx'] = peer[-1]
             srvs[peer[0]] = current
             srvlist.append(peer[0])
         r = select.select((s4, s6), (), (), 2)