Workaround for 'File exists' test failure

Adds code to create the assets directory that django_pyscss
intends to create for scss.  This avoids throwing an exception
in concurrency scenarios

Change-Id: Ic55e2a7ce59ec9b7550a7412697cb8d6563d98a3
Closes-bug: #1367590
Merge "Workaround for 'File exists' test failure"
Merge branch 'master' of https://github.com/openstack/horizon