Automatically set permissions for my.cnf

[achekulaev]

On Windows because of difference in permissions model this file has too open permissions by default. Because of that MySQL server refuses to accept it (example: #207):

Warning: World-writable config file '/etc/mysql/my.cnf' is ignored

We should set permissions to the ones that will be acceptable to MySQL during in _start_containers function.

[achekulaev]

The issue is irrelevant because my.cnf is being copied to another place and proper permissions are being set by db container itself.