Use oslo's execute() and ssh_execute() methods

Based on nova's Ice9cdbdc5f3e5a9f8365f5d99acf1863a9fe3e7a

This patch also PEP8-fy the imports from common/utils

Change-Id: I71f08e892e438ce998c1c80d15c0c6a287112d66
Merge "Use oslo's execute() and ssh_execute() methods"