Add CINDER_LOCALEDIR env variable

Add CINDER_LOCALEDIR env variable

Part of fixing bug #995287

Syncs these two commits from oslo-incubator:

  Support overriding oslo localedir too
  Add a gettextutils.install() helper function

to get a new gettextutils.install() function which allows the default
localedir to be overwritten via an environment variable.

Note that gettextutils.install() must be called before any other cinder
modules are imported since some modules attempt to translate strings
at import time (e.g. in cinder.flags). This is broken and inefficient,
but fixing it involves adding something like sphinx's l_() function and
would be very invaisve.

Change-Id: I86562b3a65d371673bb21f7179eecc7602bc0775