Fixed missing MySQL log directory

Fix to create /var/log/mysql directory which is expected by:

default["percona"]["server"]["slow_query_log_file"] = "/var/log/mysql/mysql-slow.log"

Without this MySQL server returns the following error:

140522 17:19:24 [ERROR] Could not use /var/log/mysql/mysql-slow.log for logging (error 2). Turning logging off for the whole duration of the MySQL server process. To turn it on again: fix the cause, shutdown the MySQL server and restart it.