erp5.mariadb: Explicitly disable innodb_file_per_table .

MySQL changed the default value 3 times since 5.1, causing a waste of disk
space when recreating schemas (tables move out of an already large system
tablespace, which cannot be easily reclaimed).
Also, per-table option fragments free space: if a table shrinks, freed
space cannot be used by another table without recreating shrunk table
(which induces at least a write-lock during data copy, which may not be
trivial).