Fix the logrotate file [2]
    
    In one of the previous commits, I've started to pack a different logrotate file
    as it contains the correct path to the logfile of the mysql server.
    
    It has been found, that I've overlooked that the filepath to the mysql server
    was still set wrongly.
    It was set to a:     '/var/log/mysqld.log'
    Instead of expected: '/var/log/mysql/mysql.log'
    
    This path is added to the mysql logrotate configuration file via CMAKE variable
    substitution.
    However since we haven't specify the variable value,
    a default value has been used instead.
    
    The fix to this issue was simple - specify the variable value as the CMAKE
    command argument.
    
    --
    
    At the same time, we were using a custom (downstream) variable meant to hold
    the same information. We used this variable for generating the correct filepath
    to the mysql server logfile in the mysql server configuration file.
    
    Instead of using two different CMAKE variables with the same meaning,
    I've changed it so we get rid of the downstream specified variable
    and use the upstream specified variable instead.
    
    --
    
    The positive effect is the de-duplication of code and assurance that when we
    set the value, it is used everywhere - both in the upstream code and in the
    downstream patches.
    
    The tiny negative effect is that at this moment, the names of the variables
    differ to the we use in 'mariadb' package.