Allow override default physnets

Some customers want to be able to specify list of physnets
(provider nets) explicitly, and not having default physnet1/2/3.
We already allow override 'bridge_mappings' on computes so
this is logical continuation.

Change-Id: Iff9657c5261b3b2f715a8b1fbd7dd925187a7857
Related-Prod: PROD-21121
diff --git a/tests/pillar/control_single.sls b/tests/pillar/control_single.sls
index 445806c..79894f7 100644
--- a/tests/pillar/control_single.sls
+++ b/tests/pillar/control_single.sls
@@ -18,6 +18,8 @@
       token: token
       tenant: admin
       physnets:
+        external:
+          mtu: 1500
         sriov_net:
           mtu: 9000 # Optional, defaults to 1500
           vlan_range: '100:200' # Optional