Bug #13010061/Bug # 14506305:   SERVER SHOULD WARN WHEN NEEDED AUTHENTICATION  PLUGIN IS NOT LOADED 
    
This fix fixes the above two bugs by introducing a new check at server startup and FLUSH PRIVILEGES that iterates  over the list of the user accounts, checks if one of the   conditions described in the above two bugs is met and  logs an appropriate warning in the server log.  In particular warnings will be generated for users with sha256 and server started without ssl/rsa config.
The check will slow down FLUSH PRIVILEGES and server  startup, so a new boolean command line option 
 --validate-user-plugins=[ON|OFF], default ON is added to allow turning it off for installations that don't need this additional safety net.

Test cases for various combinations of options added.
Existing test results updated as needed.