Adds unit tests for drivers.rbd.RBDImageIOWrapper

Change-Id: Id256ec6ed20c1f52eb67cf5a807f67b01a641962
Closes-bug: 1238614
Adjust import order according to PEP8 imports rule

This patch just adjust import order to follow PEP8 imports rule.
See http://www.python.org/dev/peps/pep-0008/#imports

Imports should be grouped in the following order:
1) Standard library imports
2) Related third party imports
3) Local application/library specific imports

Change-Id: Ifa3f219207b02d60e757e17c42dae1fef2f628fe
Merge "Adds unit tests for drivers.rbd.RBDImageIOWrapper"