[feature] Add possibility to enable specific commands for each organization

[nemesifier]

We need a solution that allows us to define which commands are available to each org, eg:

ENABLE_COMMANDS = {
    '__all__': ['reboot', 'ping', 'traceroute', 'change_password', 'custom'],
    '<ID openwisp>': ['reboot', 'ping', 'traceroute'],
    '<ID ACME>': '*'
}

We need to add some tests and document this feature.