XenAPI: Don't assume specific network interfaces

The default for GUEST_INTERFACE_DEFAULT now uses the ip command
to find an interface; so it will work on multiple distributions.

XenAPI should not be setting a specific interface here, as it will
almost always be wrong.  In most cases, the calculated value for
GUEST_INTERFACE_DEFAULT will be a better default.

PUBLIC_INTERFACE_DEFAULT makes even less sense as it's often an
internal bridge for devstack scenarios.

In both cases, the right way to override these is to set
GUEST_INTERFACE / PUBLIC_INTERFACE in the localrc rather than
changing the _DEFAULT values.

Change-Id: I0cf84438d778bf1a2481328165513c59167490e2
diff --git a/lib/nova_plugins/hypervisor-xenserver b/lib/nova_plugins/hypervisor-xenserver
index e7f1e87..e75226a 100644
--- a/lib/nova_plugins/hypervisor-xenserver
+++ b/lib/nova_plugins/hypervisor-xenserver
@@ -24,8 +24,6 @@
 # Defaults
 # --------
 
-PUBLIC_INTERFACE_DEFAULT=eth2
-GUEST_INTERFACE_DEFAULT=eth1
 # Allow ``build_domU.sh`` to specify the flat network bridge via kernel args
 FLAT_NETWORK_BRIDGE_DEFAULT=$(sed -e 's/.* flat_network_bridge=\([[:alnum:]]*\).*$/\1/g' /proc/cmdline)
 if is_service_enabled neutron; then