Implement API to configure API service

We provide config collection to programmatically configure Altai API
service. Configuration variables are stored in database, with values
encoded into JSON.

We also provide command-line utility to manipulate this values right in
the database, to make it possible to configure API service without API
service running.

To make access to configuration variables easier we put config callable
into flask.g. As a working and useful example of using it we implement
password reset disabling.

Change-Id: I75298f6e82e7abc5f1bb5d98446b99fa5a654ce3