ENG-6818: Initial working for DROP ROLE
ENG-7088: Users that have roles which don't exist now just log a
warning and ignore the role when computing access levels for a user.
ENG-7237: Require a user in the ADMINISTRATOR role in order to enable
security
Merge pull request #1827 from VoltDB/roletickets-final

Roletickets final, ENG-6818, ENG-7088, ENG-7237.  r=jhugg, akhanzode