Use strutils from oslo.utils

Rather than sync'ing with oslo-incubator, let's use the library
oslo.utils instead. Removing the local copy of strutils.

With removal of strutils in local, gettextutils is now safe to
remove as well.

Change-Id: I0d98d41b03d105e34933efcdb5c60020141e13fb
Partially-Implements: blueprint improve-oslo-usage
Merge "Use strutils from oslo.utils"