Add NOVA_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 nova
modules are imported since some modules attempt to translate strings
at import time (e.g. the 'message' attributes on classes in
nova.exception). This is broken and inefficient, but fixing it involves
adding something like spinx's l_() function and would be very invaisve.

Also, note that calling gettextutils.install() in nova.cmd.__init__
means that no program which uses a different translation domain should
ever import any of the modules under nova.cmd.

Change-Id: I86562b3a65d371673bb21f7179eecc7602bc0775
Merge "Add NOVA_LOCALEDIR env variable"