Fix FLOATING_IP_POOL default value

A recent change has made the "null" string a bad value for
FLOATING_IP_POOL. By making it an empty string, we recreate the old
functionality of having the default floating IP pool set to "No
value".

Change-Id: If6f2af7a8a6ccf4bb5079e8282962098f2f725b6
Signed-off-by: Eric Ball <eball@linuxfoundation.org>