Migrate to oslo.i18n

Mostly trivial import changes.

- oslo.i18n no longer provide install() method to inject _() into
  globals(), so removed all calls to it;
- removed Babel from dependencies (it will now be grabbed by oslo.i18n);
- updated tox.ini to ignore import violations for oslo.i18n.

Change-Id: I6623d551f512fb7fe9bf35ee734ed6d4c6cbc287
Merge "Migrate to oslo.i18n"