With the credentials header now available in the CI environment, build
and run the NetX Duo Azure IoT example.

The credential strings have been decoupled from `sample_config.h` into
a standalone `sample_azure_iot_credentials.h` so the CI can copy in its
own credentials header without affecting any other options in
`sample_config.h`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>