log parameter on mysqld section shouldn't be read by xtrabackup

Verified with PS 5.5.38 and Xtrabackup 2.0, 2.1 and 2.2 versions. I can able to start MySQL with --log option in my.cnf which was not possible with PS 5.6

For 2.0, getting this error.

/home/nilnandan/Downloads/percona-xtrabackup-2.0.7/bin/xtrabackup: ambiguous option '--log=/var/log/mysql/mysql.log' (log-copy-interval, log-stream)
innobackupex: fatal error: no 'mysqld' group in MySQL options
root@nilnandan-Dell-XPS:/home/nilnandan/Downloads/percona-xtrabackup-2.0.7/bin#

For 2.1 and 2.2, getting this,

Unknown suffix '/' used for variable 'log-copy-interval' (value '/var/log/mysql/mysql.log')
xtrabackup: Error while setting value '/var/log/mysql/mysql.log' to 'log-copy-interval'
innobackupex: Error: no 'mysqld' group in MySQL options at /usr/bin/innobackupex line 4397.

The workaround is to use 'general_log' instead of 'log' in my.cnf.

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-919