Mender API, UI

• support for Mender client config

root@iqube:/etc/mender# cat mender.conf

{
    "InventoryPollIntervalSeconds": 28800,
    "RetryPollIntervalSeconds": 300,
    "ServerURL": "https://mender.iqrf.org",
    "TenantToken": "dummy",
    "UpdatePollIntervalSeconds": 1800
}

• being able to enable/disable this feature via iqrf-gateway-webapp-manager
• in default webapp config disabled