update Mirantis OpenStack version to 5.1

Additional cleanups:

 * Removed duplicated explanation of Ceilometer
   performance considerations for MongoDB database backend from the
   terminology page and left it in the user guide where it belongs.

 * pt_archiver code fragment width reduced to fit inside page margins in
   PDF.

 * toctree and preface removed from pdf_relnotes.rst.

Release notes are left referring 5.0.1, that will have to be addressed
in a separate commit.

Change-Id: Ife0d774c54c603a9eb8881649bf7cbdd8345e67d