Refactor Huawei Volume driver

Refactor Huawei Volume driver

This patch attempts to refactor Huawei volume driver in liberty.
We add a base driver to implement the basic functions.
The sub-class will inherit from the base driver according to different
storages.

The following changes were made in this refactor:
1. Abstract a base class named HuaweiBaseDriver to make Huawei driver more
universal. You can find it in the huawei_driver.py.
2. Put all static variables into the constants.py.
3. Rename rest_common.py to rest_client.py. rest_client.py stores the
relevant methods implemented for Huawei driver.
4. Migrate some public methods from rest_client.py to huawei_utils.py,
such as parse_xml_file(), _get_volume_type() and so on.
5. This refactor only involves structural adjustment and does not involve
functional changes.

Change-Id: I768889e2577a4d975397218eb31e89b42e08e04f
Implements: blueprint refactor-huawei-volume-driver