Add networkd prefix when creating networks

systemd networkd is a shared directory of networks that needs to be able
to co-exist with the host which may be using networkd on it's own. This
change adds an nspawn specific networkd prefix to the interface files to
ensure we're not creating interface file collisions.

Change-Id: I1a9e25f697b64e292261cf0abf8df61a97dd2730
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>

tasks/nspawn_networking.yml

@@ -108,6 +108,7 @@
     name: systemd_networkd
     private: true
   vars:
+    systemd_networkd_prefix: "nspawn_host"
     systemd_run_networkd: true
     systemd_interface_cleanup: false
     systemd_netdevs: |-