Harden mysqlbackup.sh script

 * mysql::backup backuprotate parameter sets the number of backups to keep,
   default is 30.

 * Use bash in mysqlbackup.sh to get exit status of mysqldump when piped to
   bzip2. Unfortunately there is no easy portable way to do that.

 * Only delete old backups when current backup finished successfully.

 * Try hard not to delete files that we didn't create (i.e. README or other
   backups).