Make vagrant work behind a proxy

Without this change, I cannot get the in-tree VPP Vagrant to work behind
a proxy. This commit fixes that by ensuring when we run sudo we're passing
environment variables, amongst some other cleanups.

Change-Id: Ica98a1238d40e6e6ccf80c472f52559c95af52f0
Signed-off-by: Kyle Mestery <mestery@mestery.com>
diff --git a/extras/vagrant/clearinterfaces.sh b/extras/vagrant/clearinterfaces.sh
index 78f6705..31a6309 100755
--- a/extras/vagrant/clearinterfaces.sh
+++ b/extras/vagrant/clearinterfaces.sh
@@ -12,6 +12,6 @@
 for intf in $(ls /sys/class/net) ; do
     if [ -d /sys/class/net/$intf/device ] &&
         [ "$(route --inet --inet6 | grep default | grep $intf)" == "" ] ; then
-        ifconfig $intf down
+        sudo -E ifconfig $intf down
     fi
 done