generic_mysql: shorter delays before retries

This mysql_update service retries every 30 seconds, but what usually
happens is that it starts at same time as mariadb, make a first attempt
at connect which fails because mariadb is not ready, sleep for 30
seconds, reties and succeed. Which is not efficient because we don't
need to wait for so long.
Instead of sleeping for 30 seconds between each retries, start with a
short delay and increase each time, but not more than 30 seconds.