Added dbal.path config parameter for test environment

Parameter %sylius.database.path% was added to the parameter.yml.dist
file so that at install time it is possible to choose the data file when
using pdo_sqlite as a persistence driver.
This is especially useful in test environment when using sqlite data
file on a fast/memory filesystem (such as tmpfs), to speed up test
execution.

The parameter defaults to null (~) not to interfere with standard
configuration.