Adding possibility to change MTU on deploy_network

Change-Id: Ic58c0b664a4a0c878dd770814382cbd2e55a77f6
diff --git a/day01-image/files/root/interfaces b/day01-image/files/root/interfaces
index 5487659..97aaea3 100644
--- a/day01-image/files/root/interfaces
+++ b/day01-image/files/root/interfaces
@@ -13,6 +13,7 @@
   address $SALT_MASTER_DEPLOY_IP
   netmask $DEPLOY_NETWORK_NETMASK
   gateway $DEPLOY_NETWORK_GW
+  mtu $DEPLOY_NETWORK_MTU
   dns-nameservers $DNS_SERVERS
 #auto ens4
 #iface ens4 inet static