compat: Wrap 'render_to_string'

compat: Wrap 'render_to_string'

The 'dictionary' and 'context_instance' parameters for the
'render_to_string' function produce 'RemovedInDjango110' warnings
in Django 1.9 and will be removed in Django 1.10. However, these
parameters are still valid for Django < 1.8. Provide a shim to
allow use of the Django>=1.8-style parameters, while retaining
backwards compatibility with Django < 1.8.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Tested-by: Andy Doan <andy.doan@linaro.org>