Raise an error if iSCSI is not supported

Since the compute nodes may not have iSCSI support, volume drivers
requiring it will raise an error if the connector's initiator attribute
is not set when initializing the connection.

bug #1007084

Related to: I5da472e5013fdc0c733ecb12b919c1068873bdc6

Change-Id: I95d859c11cf81bf57cecf7af8b224cdbb71592d5