Refactor Huawei iSCSI driver

Refactor Huawei iSCSI driver

We plan to refactor Huawei iSCSI drivers and add Huawei FC
drivers. For that's a huge change, we break the codes into
three patches:
1.Refactor T iSCSI driver
2.Refactor Dorado iSCSI driver
3.Add FC drivers for both T and Dorado arrays.

This is the first patch, changes as follows:
1.Define a common class for both FC and iSCSI drivers, and
also provide a unified class HuaweiVolumeDriver for users.
The unified driver will call HuaweiTISCSIDriver according to
users' configuration. The HuaweiTISCSIDriver is a subclass of
driver.ISCSIDriver, so it could get good inheritance.
2.Support volume type.
3.Refactor unit test to make it more logic clear and add more
test cases to get higher coverage rate.

Change-Id: I79b7bac7f38f2dcbb22c5db6207d8b55906fdad1