Set force_snat_range to ANY network

Due to change 59ac254bf15bb059cca12a82c9d819c371ea5c6f
Nova-network checks whether network has external gateway
and does (which it has by default) and does not create SNAT
rules for the instances if list of force snat ranges is empty
now.

This change sets force_snat_range to ANY network and thus
new code adds corresponding rules.

Change-Id: I8d22a2564dfcd8e81543d7a25089a0e7af3ca2b3
Closes-bug: #1381164
Merge "Set force_snat_range to ANY network"