Fix Scss compilation tests

The PROJECT_ROOT directory was different based on on the current working
directory, so the tests would work if run from opentreemap/, but not
when run from other directories
Allow fab vagrant to take a path, for use in the OTM2-vagrant project
Fix spurious UI test failures from invalid ContentType cache

I found this solution at https://code.djangoproject.com/ticket/10827
Merge pull request #1830 from maurizi/fix-tests

Fix tests