Ensure container's rootfs is set on config file

Makes our lives easier as we are able to just run a `lxc-start -n CONTAINER`
to check how things are doing without having to go through vagrant.

boxes/common/lxc-template

@@ -78,6 +78,8 @@ copy_configuration()
     rootfs=$2
     name=$3
 
+    grep -q "^lxc.rootfs" $path/config 2>/dev/null || echo "lxc.rootfs = $rootfs" >> $path/config
+
     # if there is exactly one veth network entry, make sure it has an
     # associated hwaddr.
     nics=`grep -e '^lxc\.network\.type[ \t]*=[ \t]*veth' $path/config | wc -l`