implemented custom --defaults-file

Paweł Tomulikrequested to merge
ptomulik/automysqlbackup:custom-defaults-file into debian

This PR attempts to provide a better way to access DB safely. Currently there are two options, either:

  • pass USER and PASSWORS via CLI parameters, or
  • use /etc/mysql/debian.cnf as --defaults-file.

The first approach (CLI params) is considered unsecure, as the password gets exposed to other users via process list. The second approach is acceptable, but has a flaw that the /etc/mysql/debian.cnf path is a hard-coded in automysqlbackup and the /etc/mysql/debian.cnf is now OBSOLETE and will be removed in future Debian versions.

This PR makes the --defaults-file configurable. It introduces $DEFAULTSFILE variable that may be set in /etc/default/automysqlbackup. The variable is set to "/etc/mysql/debian.cnf" by default.

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