Add a gettextutils.install() helper function

Part of fixing bug #995287

Every top-level script in every project needs to call gettext.install()
so that a _() builtin function is installed and strings are translated
using the correct translation domain.

However, translations are always installed in the default localedir
(which is commonly '/usr/share/locale') and, in cases like devstack, may
be found in a project-specific localedir. To support such a use case we
should have project-specific environment variables for overriding the
default value of localedir - e.g. NOVA_LOCALEDIR.

Add a new gettextutils.install() helper method to make this as easy as
possible for projects to get right.

Change-Id: I6c8549c8ff00797c96f2dd4b0b5266d18d77aa19
Merge "Add a gettextutils.install() helper function"