xenapi: enable user interaction with stack.sh

In XenServer, devstack runs inside a virtual machine. This makes it hard
for the user to interact with stack.sh should a parameter be missing.
This change will create an upstart job (devstack) that runs
stack.sh with console enabled so user can interact with it by running
vncviewer or using XenCenter.

Logging the output is also disabled, stamp files are used instead to
detect the script run status in case install_os_domU.sh is used.

As run.sh.log is removed, standard devstack logging should be used.

The change also removes the environment settings from run.sh, as they
are not needed, they should be specified in localrc. This way user
cannot get different experiences by using unstack.sh/stack.sh or run.sh

Also a proper unstack.sh is called instead of killing screen in run.sh

Change-Id: I7eb12bd74746cc7a1db3aa9fd68ece645a50001d
xenapi: enable user to specify FLAT_NETWORK_BRIDGE

install_os_domU.sh failed, if the FLAT_NETWORK_BRIDGE is found in
localrc. As nova looks up the network by either name-label or bridge
name, it makes sense to enable the user to specify this parameter. As an
example, if the user wants to use name-labels to specify networks, and
those name-labels could be used in domU to create bridges:

VM_BRIDGE_OR_NET_NAME="osvmnet"
FLAT_NETWORK_BRIDGE="osvmnet"

In this case, the domU will know only about a name label, so it could be
decoupled from which xapi bridges used.

This change also adds some fixes (missing double quotes).

Change-Id: I045e367ef441be20c4e8cb8af3c1149392db796b
xenapi: display IP and DevStack result on console

The devstack setup service can update /etc/issue, displaying the status
of the installation and the VM's management IP. With this change, after
the devstack service finsihed, the login prompt will look like this:

OpenStack VM - Installed by DevStack
  Management IP:   10.219.3.108
  Devstack run:    SUCCEEDED

DevStackOSDomU login:

This helps people to log in to their system.

Change-Id: Idd6bbd5faf9ced5618cd3e95191bfc3b89473fa2
python-libguestfs should only be installed for libvirt/KVM.

And only when on a Redhat-based distribution

Change-Id: Ie99fd1ee54f5aaa639a3c5d8534315a4e2cac957