Fix incorrect usage of sa.String() type

In downgrade of 4eca4a84f08a_remove_ml2_cisco_cred_db migartion
there is a mistake in usage SQLAlchemy String type.
Used sa.string() instead of sa.String()

Change-Id: I521dd63ca2b48e902ca2cb17c45a3fe996b060e7
Closes-bug: #1312124
Merge "Fix incorrect usage of sa.String() type"