Set volume_driver for iscsi backend explicitly

Setting the volume_driver option explicitly even for iscsi backend is
desirable, because if the user would switch from e.g. an nfs backend
to iscsi, the volume_driver config option would not be changed during
the Puppet run and still point to the nfs driver.

Defaults to LVMISCSIDriver, can be overriden to LVMISERDriver.

Change-Id: I860508ff92fce4add21c50e684e1717fc4f16622
(cherry picked from commit 0f67be560c7b95d80555fd4fd4cd794274bf7d8a)