Add MTR option to run tests with real server defaults

It's often important, especially on the stage when we try to create a test case for a problem initially observed in a realistic environment, to run such test case with server config based on real defaults (possibly with explicit additions from the failing environment). It happens a lot that when someone runs seemingly the exact same set of statements in MTR, the failure stops happening (or vice versa), and a lot of time is spent on investigating why it is so; or even worse, the problem is declared to be not reproducible.

The goal of this task is to add a command-line option --default-server-options (with no argument) to mysql-test-run.pl which will

• replace usual default_my.cnf, containing numerous non-default options, with an empty config (apart from locations and paths which still need to be there, of course); the empty config template is also added in the scope of this task;
• disable skipping optional plugins which happens in My/ConfigFactory.pm;
• remove hardcoded additions to bootstrap.

The typical use of the option for developers would be, when a reporter posts a set of SQL commands to reproduce a bug, rather than an MTR test case (which is usual for external reporters), and when the failure does not occur when the developer puts this set of SQL commands into an MTR test case, try running it with --default-server-options and see if it makes a difference.

Testers will have more use for the option.

The option only affects "default defaults". Whatever is added through custom cnf files and/or opt files will still be added.