Switch over oslo.i18n

oslo.i18n has recently been released. This patch switches glance over
oslo.i18n. As per oslo.i18n's instructions, a new i18n module has been
added under glance. This module defines the translation globals and
imports the necessary functions from oslo.i18n.

The patch doesn't change the way Glance does translation, this means
that a `_` function is still being injected to the builtins.

Note that the gettextutils module is still a required module from
oslo-inc because there are oslo-inc modules that depend on the old
gettextutils module.

Change-Id: Ieb83681e35f72a5a4ecd849bb76af7f78d9fdcb0
Merge "Switch over oslo.i18n"