Feature: Set Logging:LogLevel:System to Warning by default

[kbeaugrand]

Is your feature request related to a problem? Please describe.
When deploying the portal, we must ensure that by default the configuration should be sufficient to diagnose and also guaranty that no credentials are present in logs.
An issue was created on the LoRaWAN Starter kit revelling that the facade auth code is present on the logs. We must use the suggestion to our portal deployment.

Describe the solution you'd like
Set default log level to warning for:
- System.Net.Http.HttpClient
- Microsoft.Extensions.Http.DefaultHttpClientFactory

Describe alternatives you've considered
N/A

Additional context
See Azure/iotedge-lorawan-starterkit#1916 for more information.