Remove gettext.install() from nova/__init__.py

The gettext.install() function installs a builtin _() function which
translates a string in the translation domain supplied to the install()
function. If gettext.install() is called multiple times, it's the last
call to the function which wins and the last supplied translation domain
which is used e.g.

 >>> import os
 >>> os.environ['LANG'] = 'ja.UTF-8'
 >>> import gettext
 >>> gettext.install('keystone', unicode=1, localedir='/opt/stack/keystone/keystone/locale')
 >>> print _('Invalid syslog facility')
 無効な syslog ファシリティ
 >>> gettext.install('nova', unicode=1, localedir='/opt/stack/nova/nova/locale')
 >>> print _('Invalid syslog facility')
 Invalid syslog facility

Usually this function is called early on in a toplevel script and we
assume that no other code will call it and override the installed _().
However, in Nova, we have taken a shortcut to avoid having to call it
explicitly from each script and instead call it from nova/__init__.py.

This shortcut would be perfectly fine if we were absolutely sure that
nova modules would never be imported from another program. It's probably
quite incorrect for a program to use nova code (indeed, if we wanted to
support this, Nova code shouldn't use the default _() function) but
nevertheless there are some corner cases where it happens. For example,
the keystoneclient auth_token middleware tries to import cfg from
nova.openstack.common and this in turn causes gettext.install('nova')
in other projects like glance or quantum.

To avoid any doubt here, let's just rip out the shortcut and always
call gettext.install() from the top-level script.

Change-Id: If4125d6bcbde63df95de129ac5c83b4a6d6f130a
Merge "Remove gettext.install() from nova/__init__.py"