Explicitly import _() in Cinder code

To ensure that logs are properly translated and logged to
the Cinder log files (using the Cinder message catalogs)
we need to explicitly import _() in any python files that
use the _() function.

Closes-bug: 1306275
Related-Blueprint: i18n-enablement
Change-Id: I3cf1f1d63d5f8bd8ffd007104691f914e3fd8538