Fix tgt target in cinder element

The package install version creates a file at /etc/tgt/conf.d/cinder.conf
and the cinder element has been adding an additional file. This would have
the effect of the source install working properly, but the package install
would end up creating volumes in /etc/cinder/volume instead of /mnt/state.

This patch changes the tgt target to be written to the existing file to
bring both the source and package installs to behave the same.

Change-Id: Ieeaa9518d6ed50595fbd70f1bbf0a17e3b5b1894
Merge "Fix tgt target in cinder element"