Port to use oslo.i18n

Module gettextutils from oslo-incubator was graduated to separate
library (oslo.i18n) and removed from repository, so we should use
new library.

Change-Id: I15d36ac5a2bb88b332ffa38d29488b75c55a20a9
Sync latest code

Modules from oslo-incubator already switched to use graduated
libraries (oslo.i18n, oslo.serialization and oslo.utils), so we
should sync latest code and remove outdated modules (gettextutils,
importutils, strutils).

NOTE:
 - module novaclient.openstack.common._i18n should be used only in
   common code
 - module novaclient.i18n designed for usage in novaclient

Latest commit in oslo-incubator:
  21985931a95981eabf1030ead60ae4d210bcd9c0

Synced changes for tools/install_venv_common.py:
  fe3389e Improve help strings

Other synced changes:
  55ca7c3 Split cliutils
  5d40e14 Remove code that moved to oslo.i18n
  3edbfb3 remove caching param from prettytable call
  6ff6b4b Switch oslo-incubator to use oslo.utils and remove old modules
  9eee5d0 Merge "Delete the token and endpoint on expiry of token of client"
  f76f44c Delete the token and endpoint on expiry of token of client
  ed0ffb8 Do not incur the cost of a second method call
  cf449e2 Fix response_key parameter usage in BaseManager
  d73f3b1 Remove unused/mutable default args
  e7d26a6 Merge "Centralize bash-completion in Novaclient"
  b4e098e Merge "Handle non-openstack errors gracefully"
  5e00685 Centralize bash-completion in Novaclient
  4ef0193 Handle non-openstack errors gracefully
  ac995be Fix E126 pep8 errors
  de4adbc pep8: fixed multiple violations

Change-Id: I5f3015ebcbc665a2089ec88629e15ba336aee504