When acting as a Galera receiver node, server startup may take
more than 30 secs (the current default) as it has to wait for
SST/IST operation to complete besides spending some time doing
wsrep recovery.

Fixed by raising the default value of MYSQLD_STARTUP_TIMEOUT
to 60 secs. Also sourced /etc/default/mariadb into the init
script so that it can be used to set MYSQLD_STARTUP_TIMEOUT.