Automatically setup bootstrap for rocksdb tests

[maykov]

rocksdb tests are skipped unless mtr is started like this:
--mysqld=--default-storage-engine=rocksdb --mysqld=--skip-innodb --mysqld=--default-tmp-storage-engine=MyISAM --mysqld=--rocksdb

Update mysql-test-run.pl so that it is configurable. It should probably read bootstrap parameters from bootstrap.cnf in the suite root directory. If two suites have different cnf, bootstrap mysql for each.

This might be pretty involved task, reqiures some Perl knowledge.