The LMS was overriding CMS_BASE properly, but Studio (CMS) configuration
was not. That meant that Studio's CMS_BASE in dev mode was using the
devstack default of localhost:18010 (because this is what's defined in
edx-platform). This in turn broke parts of Studio that use this value,
such as the XBlock v2 API (/api/xblock/v2).
This commit derives the value of the CMS_BASE Django setting from
Tutor's CMS_HOST config value, in the same way that the LMS does it.
