Always split template names on forward slash

Template names in Django always use forward slash, even on Windows,
so to parse them properly, split on forward slash instead of
os.path.sep which is '\\' on Windows.

Change-Id: Ib72b379397f014aa1e4557fed02cbac5d931b84e
Closes-Bug: #1371610
Co-Authored-By: Timur Sufiev <tsufiev@mirantis.com>