Fix and clean up various gettext issues

The following needs to be done to resolve this:
1. [Urgent] Proposed a fix for the issue in http://logs.openstack.org/25815/2/gate/gate-ceilometer-python27/1914/console.html.gz to make the jenkins gate job happy.

2. Remove gettext.install() from ceilometer/__init__.py and move that to top level scripts, so that importing ceilometer's module(i.e. ceilometer.compute.nova_notifier) in other project(i.e. nova) won't override its parenting project's _() function. See nova patch https://review.openstack.org/25823 for reference. Maybe we should wait until the oslo patch https://review.openstack.org/25820 arrived.

3. Modify the ceilometer.compute.nova_notifier so that it has its own _() function for i18n instead of using the one in python build-in namespace registered by the parenting project's gettext.install().

Gerrit topic: https://review.openstack.org/#q,topic:bp/gettext-i18n-issue,n,z

Addressed by: https://review.openstack.org/25883
    Adpated to nova's gettext changes.

Addressed by: https://review.openstack.org/25983
    Update part of openstack.common.

Addressed by: https://review.openstack.org/25984
    Prepare for future i18n use of _() in nova notifier.

Addressed by: https://review.openstack.org/25985
    Remove gettext.install from ceilometer/__init__.py

Addressed by: https://review.openstack.org/26210
    Adpated to nova's gettext changes.