Add helper method total_seconds in timeutils.py

Almost every OpenStack project is using hardcode to calculate duration
like 'delta.days * 24 * 3600', even 2592000. Method delta_seconds takes
a timedelta instance as input to compute total seconds.

Method delta_seconds also is refactored to call new method.

Change-Id: I397bae402e012c28aaa6aa2bb10170b83eeeaa8c
Merge "Add helper method total_seconds in timeutils.py"