fix: errors adding/upgrading data into couchbase persistence

[iromli]

Description

The changeset fixes 2 errors found while using Couchbase persistence:

1. Some entries are placed into incorrect bucket
2. Loading role mapping from persistence throws TypeError

Implementation Details

1. Trailing comma must be added into the tuple (i.e. ("tokens_",) instead of ("tokens_")) to ensure key prefix is checked against a tuple (not a list of characters)
2. Load role mapping from persistence using json.loads if it's a JSON string, otherwise use the role mapping directly