Fix iscsi_write_cache setting for iscsi targets

Fix iscsi_write_cache setting for iscsi targets

While transitioning to the new driver and target
model (change 9651f547147188645942466602c92cce06666483)
some things got lost not surprisingly.  One of those
things that wasn't migrated correctly was the
iscsi_write_cache option due to ensure_export not
being called properly on service restart.

This patch cleans up the ensure/create_export methods
and their signatures.  Most importantly esnure_export
should now actually work and check the iscsi_write_cache
settings.

While fixing this I also cleaned up the two methods
mentioned above to eliminate the unnecessary and
duplicate info in their arguments.

Closes-Bug: #1408171

Change-Id: Ibfd1feefd72c43ef316b267e9d6645f2e67e2558