config: bring back sqlite default database (apache#3955)

That became postgres in 268edcf

Fix apache#3954

superset/config.py

@@ -58,9 +58,9 @@
 SECRET_KEY = '\2\1thisismyscretkey\1\2\e\y\y\h'  # noqa
 
 # The SQLAlchemy connection string.
-# SQLALCHEMY_DATABASE_URI = 'sqlite:///' + os.path.join(DATA_DIR, 'superset.db')
+SQLALCHEMY_DATABASE_URI = 'sqlite:///' + os.path.join(DATA_DIR, 'superset.db')
 # SQLALCHEMY_DATABASE_URI = 'mysql://myapp@localhost/myapp'
-SQLALCHEMY_DATABASE_URI = 'postgresql://root:password@localhost/myapp'
+# SQLALCHEMY_DATABASE_URI = 'postgresql://root:password@localhost/myapp'
 
 # In order to hook up a custom password store for all SQLACHEMY connections
 # implement a function that takes a single argument of type 'sqla.engine.url',