Improvements in csv export for usage data

Added a csv writer using the 'csv' library to format
properly exported data - escaping, encoding etc.
Added a HttpResponse-based class to handle csv generation
Added translation for the CSV columns and template.

Fix bug #1158383

Renamed few occurencies of 'tenant' to 'project'.

Also added a new 'project' in nova_data.py, which required
some refactoring of few tests, that didn't consider the current project
for project-based calls.

Note: I've added a StreamingHttpResponse example,
which is introduced in Django 1.5+ and being advised in the ticket.
However, my opinion is that at the moment we don't need this - it is
too complicated for the current usage.

Change-Id: I18bd70a23b7b8389c7c0f96dbf8794fea5e1e75b
Merge "Improvements in csv export for usage data"