Revert "Revert "use mysql db""

This reverts commit e68772fe1790f4ed351980693f6cd7113dc87510.
Revert "Revert "add instructions for mysql version""

This reverts commit 9404276e3d5bcf0cfbf6d5e25bf8bb4bb4f7ae2f.
openZIM: added installation instr. for ubuntu