Enable bridge firewalling if iptables are used

With the plan [1] to stop enabling it by Neutron iptables firewall
driver itself, deployment tools should catch up and enable the firewall
themselves.

This is needed for distributions that decided to disable the kernel
firewall by default (upstream kernel has it enabled). This is also
needed for distributions that ship newer kernels but don't load the
br_netfilter module before starting nova-network or Neutron iptables
firewall driver. In the latter case, firewall may not work, depending on
the order of operations executed by the driver.

To isolate devstack setups from the difference in distribution
kernel configuration and version, the following steps are done:

- we load bridge kernel module, and br_netfilter if present, to get
  access to sysctl knobs controlling the firewall;
- once knobs are available, we unconditionally set them to 1, to make
  sure the firewall is in effect.

More details at:
http://wiki.libvirt.org/page/Net.bridge.bridge-nf-call_and_sysctl.conf

[1] I9137ea017624ac92a05f73863b77f9ee4681bbe7

Change-Id: Id6bfd9595f0772a63d1096ef83ebbb6cd630fafd
Related-Bug: #1622914
diff --git a/lib/nova b/lib/nova
index 334cba6..d321b73 100644
--- a/lib/nova
+++ b/lib/nova
@@ -866,9 +866,13 @@
     run_process n-cond "$NOVA_BIN_DIR/nova-conductor --config-file $compute_cell_conf"
     run_process n-cell-region "$NOVA_BIN_DIR/nova-cells --config-file $api_cell_conf"
     run_process n-cell-child "$NOVA_BIN_DIR/nova-cells --config-file $compute_cell_conf"
-
     run_process n-crt "$NOVA_BIN_DIR/nova-cert --config-file $api_cell_conf"
+
+    if is_service_enabled n-net; then
+        enable_kernel_bridge_firewall
+    fi
     run_process n-net "$NOVA_BIN_DIR/nova-network --config-file $compute_cell_conf"
+
     run_process n-sch "$NOVA_BIN_DIR/nova-scheduler --config-file $compute_cell_conf"
     run_process n-api-meta "$NOVA_BIN_DIR/nova-api-metadata --config-file $compute_cell_conf"