Skips enabling kernel bridge firewall in container

Calling enable_kernel_bridge_firewall inside a
container, devstack will crash because it tries to
load a kernel module by calling 'sudo modprobe' on
net.bridge.

Change-Id: Id4718c065d5a8c507d49f38e19c2796a64221aa4
Closes-Bug: #1662194
diff --git a/lib/nova b/lib/nova
index 4d336f6..d1c0d47 100644
--- a/lib/nova
+++ b/lib/nova
@@ -864,7 +864,9 @@
     run_process n-crt "$NOVA_BIN_DIR/nova-cert --config-file $api_cell_conf"
 
     if is_service_enabled n-net; then
-        enable_kernel_bridge_firewall
+        if ! running_in_container; then
+            enable_kernel_bridge_firewall
+        fi
     fi
     run_process n-net "$NOVA_BIN_DIR/nova-network --config-file $compute_cell_conf"