Change "allow-hotplug" network setting into "auto" to get more consistent behaviour

I sometimes get a failing network setup in Vagrant's ce-trunk VM
where the default route is missing then:

| # ip r
| 10.0.2.0/24 dev eth0  proto kernel  scope link  src 10.0.2.15
| 192.168.88.0/24 dev eth1  proto kernel  scope link  src 192.168.88.234
|
| # cat /etc/network/interfaces
| # This file describes the network interfaces available on your system
| # and how to activate them. For more information, see interfaces(5).
| # The loopback network interface
| auto lo
| iface lo inet loopback
|
| # The primary network interface
| allow-hotplug eth0
| iface eth0 inet dhcp
| #VAGRANT-BEGIN
| # The contents below are automatically generated by Vagrant. Do not modify.
| auto eth1
| iface eth1 inet dhcp
|     post-up route del default dev $IFACE
| #VAGRANT-END

Of course the default route comes back on a manual refresh of eth0:

| # ifdown eth0
| # ifup eth0
| # ip r
| default via 10.0.2.2 dev eth0
| 10.0.2.0/24 dev eth0  proto kernel  scope link  src 10.0.2.15
| 192.168.88.0/24 dev eth1  proto kernel  scope link  src 192.168.88.234

The culprit seems to be the allow-hotplug where the device is handled
different in the startup process:

  http://screeni.s3.amazonaws.com/screenshot.2013-09-17T11:43:37.png

The ordering is important because of Vagrant's
"post-up route del default dev $IFACE". While there's a workaround
available within Vagrant:

  73c8299ecd

I don't think that's the proper way. Since we're using "auto"
everywhere else in our setups anyway let's make this change,
hopefully also fixing the Vagrant network issue.

deployment.sh

@@ -1391,7 +1391,7 @@ auto lo
 iface lo inet loopback
 
 # The primary network interface
-allow-hotplug $EXTERNAL_DEV
+auto $EXTERNAL_DEV
 iface $EXTERNAL_DEV inet dhcp
 EOF
   # make sure internal network is available even with external