Set default value and configurable value for integration bridge

When using XenServer as hypervisor, install_os_domU.sh will create
integration bridge for compute node when neutron network is used.
But it should provide a way to allow moving of the VM to another
host (with a different XEN_INTEGRATION_BRIDGE) for easier install.
This patch is to provide the way to let user have the chance to
configure integration bridge themselves

Change-Id: If923a5e978e77fc091d24b6e1fe7a83a3375da09
diff --git a/tools/xen/install_os_domU.sh b/tools/xen/install_os_domU.sh
index 46ff0b6..8b97265 100755
--- a/tools/xen/install_os_domU.sh
+++ b/tools/xen/install_os_domU.sh
@@ -299,10 +299,10 @@
 # kernel parameter for DomU
 attach_network "$XEN_INT_BRIDGE_OR_NET_NAME"
 
-XEN_INTEGRATION_BRIDGE=$(bridge_for "$XEN_INT_BRIDGE_OR_NET_NAME")
+XEN_INTEGRATION_BRIDGE_DEFAULT=$(bridge_for "$XEN_INT_BRIDGE_OR_NET_NAME")
 append_kernel_cmdline \
     "$GUEST_NAME" \
-    "xen_integration_bridge=${XEN_INTEGRATION_BRIDGE}"
+    "xen_integration_bridge=${XEN_INTEGRATION_BRIDGE_DEFAULT}"
 
 FLAT_NETWORK_BRIDGE="${FLAT_NETWORK_BRIDGE:-$(bridge_for "$VM_BRIDGE_OR_NET_NAME")}"
 append_kernel_cmdline "$GUEST_NAME" "flat_network_bridge=${FLAT_NETWORK_BRIDGE}"