Allow users to run 'tox -epy34'

Allow users to run 'tox -epy34'

With this commit, it is possible to successfully run 'tox -epy34', even though
only a small amount of tests will actually be run. This is a required step in
making Neutron compatible with Python 3, as described in the 'Porting to Python
3' specification.

This commit:
- fixes some broken imports, while making sure they still work with Python 3;
- updates a call to gettext.install;
- adds a py34 target in tox.ini.

Change-Id: I91cc7a992d05ea85f7004d1c5a45a1c02cbf1c85
Blueprint: neutron-python3