Add support for open-iscsi transports.

This enables use of software or hardware transports viz. be2iscsi,
bnx2i, cxgb3i, cxgb4i, qla4xx, iser and ocs. The default transport
(tcp) happens to be called "default".

Use of non-default transports changes the disk path to the following format:
/dev/disk/by-path/pci-<pci_id>-ip-<portal>-iscsi-<iqn>-lun-<lun_id>

pkg/api/types.go

@@ -449,6 +449,8 @@ type ISCSIVolumeSource struct {
 	IQN string `json:"iqn,omitempty"`
 	// Required: iSCSI target lun number
 	Lun int `json:"lun,omitempty"`
+	// Optional: Defaults to 'default' (tcp). iSCSI interface name that uses an iSCSI transport.
+	ISCSIInterface string `json:"iscsiInterface,omitempty"`
 	// Required: Filesystem type to mount.
 	// Must be a filesystem type supported by the host operating system.
 	// Ex. "ext4", "xfs", "ntfs"