Enhance delayed message translation when _ is imported

Delayed message translation currently doesn't work for
messages written from files that import the _ function
directly.  For example:

from ceilometer.openstack.common.gettextutils import _

This change uses new support added to oslo that enables
lazy text translation in files where _ is imported
directly.

Implements bp user-locale-api

Change-Id: Ib2189836c6bbeb7e242a77415424fa77af5366f5
Merge "Enhance delayed message translation when _ is imported"