Use oslo.i18n

oslo.i18n provides the i18n function that were provided by
oslo-incubator's gettextutils module

Import _ where needed, oslo.i18n deprecated the builtin method.

Closes-Bug: #1382187
Change-Id: I12aa1c725aa4bb52a9aa46e9c3d2b303839de48b
Merge "Use oslo.i18n"