Fix creation of iscsi targets

Fix creation of iscsi targets

Previously when creating iscsi volumes, we were using

tgt-admin -e -c <config file> --update volume-id

Unfortunately the side affect of this is that tgt-admin
removed other volumes that weren't connected to an iscsi
connector. Which is obvlously not what we want.

In order to fix this we create the targets.conf for the
volume but we call tgt-admin --update icssi qualified name.

We're dropping the use of iscsi_targets table when using TgtAdm.
Compatability for other target admin types is maintained.

Fixes LP: #1038062

Change-Id: I9060a43208df5b79e9b17dadcab8bc0a8eeef55e
Signed-off-by: Chuck Short <chuck.short@canonical.com>