PersistentGraph/VersionedGraph hard-code parameters for create and identifier

[curtosis]

In persistent.py PersistentGraph and VersionedGraph hard-code parameters that were previously usable:

• the identifier parameter used by rdflib-sqlalchemy is set to "brickschema_persistent_graph" and "my_store", respectively
• create=True when opening the graph.

This makes it impossible to connect to existing repositories created with a different identifier, or to recognize that a repository expected to exist does not.