Network Interface should be ethernet on CI, and default interface otherwise

[simonbutcher]

Targets may not always have ethernet as their default network interface, and may instead have Wifi.

Currently the Jenkins CI always uses the default interface and assumes that will be ethernet, which may break the CI for some targets.

Therefore, the example needs to be adapted to prefer ethernet on the CI, but otherwise go with the default interface.

See PR #222 for further discussion of the issue.

[ciarmcom]

ARM Internal Ref: IOTSSL-2703