devstack: Fix neutron configuration to run in OSIC

We can merge this temporary fix and track the following
related bugs to revert. For details for the issue, please
refer to the bug descriptions.

Related-Bug: #1628267
Related-Bug: #1629133
Change-Id: I51feb6ccdc0fab91a591568866e6801f2bbb319b

devstack/settings

@@ -5,6 +5,9 @@
 # PUBLIC_NETWORK_GATEWAY=192.168.1.225
 # PUBLIC_INTERFACE=em1
 # FIXED_RANGE=10.0.0.0/24
+# FIXME: Add FIXED_RANGE until devstack/neutron is fixed globally
+SUBNETPOOL_PREFIX_V4=$FIXED_RANGE
+
 ## Log all output to files
 # LOGFILE=$HOME/devstack.log
 # SCREEN_LOGDIR=$HOME/logs