engine: Disable iSCSI multipathing for datacenters < 3.4

I had to fix a few issues in VDSM 3.4 in order to support configuration of
iSCSI multipathing, therefore this feature should be disabled for earlier
datacenter versions.

Change-Id: I623cd917ca0871a9d3d09c2d30ec2dfc114d383e
Bug-Url: https://bugzilla.redhat.com/1078137
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>