xenapi: cleanup VM Installation

VM installation created a machine with multiple network interfaces. This
is a needless complexity at that point, we only need one interface. This
change removes the complex network interface setup from
install-os-vpx.sh script, so that only one network interface is created,
which is connected to the selected network (management by default). This
change also removes a lot of non-used code from install-os-vpx.sh
Specify the network to be used for ubuntu netinstall with:

    UBUNTU_INST_BRIDGE_OR_NET_NAME

defaults to $MGT_BRIDGE_OR_NET_NAME.

This change also includes:
 - use precise as default

Related to blueprint xenapi-devstack-cleanup

Change-Id: Ib0a0a316e849f2c2651305ea657c84820cd2bb3b
diff --git a/tools/xen/scripts/install_ubuntu_template.sh b/tools/xen/scripts/install_ubuntu_template.sh
index b7a8eff..f24e7b2 100755
--- a/tools/xen/scripts/install_ubuntu_template.sh
+++ b/tools/xen/scripts/install_ubuntu_template.sh
@@ -53,7 +53,7 @@
 pvargs="-- quiet console=hvc0 partman/default_filesystem=ext3 \
 console-setup/ask_detect=false locale=${UBUNTU_INST_LOCALE} \
 keyboard-configuration/layoutcode=${UBUNTU_INST_KEYBOARD} \
-netcfg/choose_interface=${UBUNTU_INST_IFACE} \
+netcfg/choose_interface=eth0 \
 netcfg/get_hostname=os netcfg/get_domain=os auto \
 url=${preseed_url}"