Do database backup under unprivileged user

[php-coder]

We can create special user backup for doing backups. He will have limited amount of permissions and also we can use its password in ~/home/mystamps/.my.cnf file.

The following steps should be done:

• backup user in MySQL should be created (with limited access: only from localhost and it should have permissions that allows to doing backup only)
• /data/mystamps/.my.cnf file should be updated on the server with new login/password
• /etc/cron.d/mystamps should be modified to use backup user
• mystamps-backup Ansible role should be updated to reflect this changes
• instruction should be updated

Tech debt for: #116

Quote from mysqldump documentation:

mysqldump requires at least the SELECT privilege for dumped tables, SHOW VIEW for dumped views, TRIGGER for dumped triggers, and LOCK TABLES if the --single-transaction option is not used. Certain options might require other privileges as noted in the option descriptions.