Add persistent volumes for tgtd.

Add persistent volumes for tgtd.

Currently if you restart the server running nova-volume
or restart tgt, you will loose your iscsi targets that
have been created. This is not good.

In order for iscsi targets to be persistent across
reboots or restarts, one has to have the target's configuration
information in /etc/tgt/targets.conf or /etc/tgt/conf.d.
So when tgtd is restarted then the iscsi targets will be there
as expected.

This patch will add a configuration file to $state_path/volumes
when the volume is created. The configuration file is identified by
the volume uuid. It creates a logicalunit when the volume is created
as well. The iscsi target and configuration file
will be removed once the volume has been removed as well.

In order to use this, you have to include the following in
your /etc/tgt/targets.conf

include $state_path/volumes/*

For upgrades, it will just re-create the volumes
already in the volumes table.

Fixes LP: #1011159

Change-Id: Ib74dcc1efa0332842041b4c045ca5fa85d4a32ca
Signed-off-by: Chuck Short <chuck.short@canonical.com>