Add support for MongoDB for property read-write

[VigneshVSV]

There are certain settings in hardware which are helpful to be backed up to databases as the hardware "forgets" the setting when power cycled. It may also go to a default setting.

Since settings are generally modelled as properties, after the property write, the value can be stored to the database.

Currently SQLite, MySQL/postgres and JSON file storage is supported.

It's useful to have a JSON compatible DB as well.

Involved tasks -

• subclass ThingDB for mongo
• implement all operations which are imported for SQLite or other databases
• support auto-setup in case use_default_db is set to True at Thing.__init__
• verify working at Property._post_value_set
• write tests or add it to the tests in test_property.py