Support overriding oslo localedir too

Part of fixing bug #995287

Libraries need to translate strings according to their translation
domain but they should not override the default _() builtin (which
should only be installed by the top-level script) and instead should
gettextutils._().

To support the case where message catalogs are installed in a
non-default (and perhaps project-specific) location, we allow the
default localedir to be override with a project-specific environment
variable e.g. QUANTUMCLIENT_LOCALEDIR.

The code makes it seem like OSLO_LOCALEDIR is the env variable we're
adding but, in fact, update.py magically replaces 'oslo' with the
project name.

Change-Id: I62b66892a4e27892ee488d2884ffd2f40ab467ee
Merge "Support overriding oslo localedir too"