Create the /images directory used by the resize functionality.

Change-Id: I13b39dbf5537ad45160c1af4cc10bd867b7f89c1
xenapi: separate disk for cinder volumes

Some kernels have lockup issues while serving cinder volumes from
file-loopback-lvm kind of cinder setup. This patch provides a way to
create an additional drive to the OpenStack VM, and use that to store
cinder volumes, thus eliminating the lockup issue. It will help when
testing XenServer. Now, you can specify devices for stack-volumes
through:

    VOLUME_BACKING_DEVICE

In case you are using CINDER_MULTI_LVM_BACKEND, you can use

    VOLUME_BACKING_DEVICE2

as well.

Xenserver:
Should you whish to use a 10 gig disk backend for your cinder volumes,
specify:

    XEN_XVDB_SIZE_GB=10
    VOLUME_BACKING_DEVICE=/dev/xvdb

Citrix is using this approach on its internal CI system to run tests
against OpenStack. This is a workaround for this bug:

  https://bugs.launchpad.net/cinder/+bug/1023755

Related to blueprint xenapi-devstack-cleanup

Change-Id: Iee633d2704185bfbf9234882654c47b850fa168a
xenapi: Get rid of internal xapi interface

OpenStack VM had 4 interfaces, from which eth0 was not used at all. This
change removes the non-used interface, thus devstack is providing the
same architecture as described in the official documentation:

    http://goo.gl/dmrNF

Related to blueprint xenapi-devstack-cleanup

Change-Id: I73c9751ade6f4786b8b682a1994cb87f0a624379
xenapi: cleanup VM Installation

VM installation created a machine with multiple network interfaces. This
is a needless complexity at that point. This change removes the complex
network setup from install-os-vpx.sh script, so that only one network
interface is created, which is connected to the selected network. 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
 - update readme file, as network defaults were updated previously

Related to blueprint xenapi-devstack-cleanup

Change-Id: Ib0a0a316e849f2c2651305ea657c84820cd2bb3b