No support for client-side TLS

[tonyespy]

Version: 1.2.1

The current version of device-mqtt only supports server-side TLS, it's not possible to configure a local certificate that the client would use to authenticate to the server instead of using user/password authentication.

It looks like the app-functions-sdk already has support for this, so we should leverage if/when this issue is addressed:

https://docs.edgexfoundry.org/1.2/microservices/application/BuiltIn/#mqtt

[iain-anderson]

Depends on edgexfoundry/device-sdk-go#564