This patch is part of preparations for dropping 'cassandra::cassandra'
default superuser. When that is implemented, maintenance_socket_role_manager
will have two modes of work:
1. in maintenance mode, where role operations are forbidden
2. in normal mode, where role operations are allowed
To execute the role operations, the node has to join a cluster.
In maintenance mode the node does not join a cluster.
This patch lets maintenance_socket_role_manager know if it works under
maintenance mode and returns appropriate error message when role
operations execution is requested.
Refs SCYLLADB-409
b68656b59f