Have genenv offer some hepful hints about FLAT_INTERFACE

The FLAT_INTERFACE is for vm communication, and should be
manually configured if necessary.

Change-Id: I79846705c18c2f0f2d4340e53d04183ec689e191

tools/genenv

@@ -112,3 +112,6 @@ RABBITMQ_USER=$RABBIT_USER
 RABBIT_PASSWORD=$RABBIT_PASSWORD
 RABBIT_USERID=$RABBIT_USER
 EOF
+echo Please customize your FLAT_INTERFACE to a different network then your
+echo main network. The FLAT_INTERFACE is used for inter-VM communication.
+echo the FLAT_INTERFACE should not have an IP address assigned.