link /etc/tgt/stack.d to cinder volumes directory

This creates /etc/tgt/stack.d as a symlink to the cinder volumes
directory and includes it in /etc/tgt/targets.conf in a similar way
to /etc/tgt/conf.d

targets.conf only gets appended to when the symlink is created.
Any old /etc/tgt/conf.d/stack.conf is deleted

Change-Id: Ifb6412f6860ceb28b724644fb9b618638f552102
Fixes: bug #1072121
Merge "link /etc/tgt/stack.d to cinder volumes directory"