Add IPv6 support to openrc files

Assumes devstack was configured with SERVICE_IP_VERSION in
local.conf

SERVICE_IP_VERSION is stored in .stackenv and checked in
openrc. If SERVICE_IP_VERSION is set to 6, openrc will use
IPv6.

NOTE: At first, I added a '-6' option to the openrc call
which would set the HOSTS accordingly. I then simplified
the code by saving SERVICE_IP_VERSION to the .stackenv file
which is sourced by openrc. After that, I simplified the
code even more by removing an extra, unnecessary, variable.

Change-Id: I5d46d5438d3e56fea788720ca17f0010caef3df1
diff --git a/functions-common b/functions-common
index 39c1bfc..fc17d42 100644
--- a/functions-common
+++ b/functions-common
@@ -47,7 +47,7 @@
 STACK_ENV_VARS="BASE_SQL_CONN DATA_DIR DEST ENABLED_SERVICES HOST_IP \
     KEYSTONE_AUTH_PROTOCOL KEYSTONE_AUTH_URI KEYSTONE_SERVICE_URI \
     LOGFILE OS_CACERT SERVICE_HOST SERVICE_PROTOCOL STACK_USER TLS_IP \
-    HOST_IPV6"
+    HOST_IPV6 SERVICE_IP_VERSION"
 
 
 # Saves significant environment variables to .stackenv for later use