Improve Galera scripts

Many Galera scripts assumes that my_print_defaults and other MariaDB
binaries are in PATH. This might not be true.

• We can't assume that "mysql" is in the path. We have to search for this like
  we do with my_print_defaults !
• Checking for version number doesn't guarantee that the binaries are
  compiled with galera.
  Instead you should check if a binary supports any of the required wresp
  options.

You can do this with:
mysql --test-some-option --version
This will fail with an error if the option is not supported.