Add a temporary fix for the storage config.

[idimopoulos]

This PR might not be an good solution as the fields can still be created through yaml files.
#8 has a more proper solution for this case but takes a bit more time.

[idimopoulos]

Regarding the fact that I am returning the condition without throwing an error or an exception:
Under normal circumstances there should be an error/warning/exception thrown if there is an unmapped field attempted to be added in the query. The reason that I am not doing this is that unlike mysql, we are querying the for the entities in the deleteBeforeInsert method. In that case, if an invalid field is added to the RdfEntity['create'] then there will be an invalid error thrown as normally the value should just be ignored.

This should also be handled in issue #8. The create method should filter invalid values.