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