engine: docs internal server error

- If you try to load a manual document in a locale that
  doesn't exist, the server would throw a 500 due to a
  problem in the setup of path of the DocsServlet. This
  patch fixes that issue.

Change-Id: I3036d05e736a4bc2603423fb7041910a9daa1472
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1083528
Signed-off-by: Alexander Wels <awels@redhat.com>