Allow provider network to be used for ssh validation

Currently devstack assumes that the network used for ssh
validation is the private network. This patch adds a hook that
sets the network used for ssh validation based on whether or not
provider networking is being used. It also moves the function
'is_provider_network' into functions-common as it will now be
used by both tempest and neutron.

Change-Id: I265c9e26c9bfb18b7e201f27d8912b8bec235872
diff --git a/functions-common b/functions-common
index 8e14b5e..56d0a64 100644
--- a/functions-common
+++ b/functions-common
@@ -2260,6 +2260,14 @@
     echo $subnet
 }
 
+function is_provider_network {
+    if [ "$Q_USE_PROVIDER_NETWORKING" == "True" ]; then
+        return 0
+    fi
+    return 1
+}
+
+
 # Return the current python as "python<major>.<minor>"
 function python_version {
     local python_version