Hostname reporting alignment

[gregkalapos]

From elastic/apm#517

Currently we use Dns.GetHostName(). We already saw this failing in Kubernetes and according to elastic/apm#517 this reports the wrong info.

An alternative would be Environment.MachineName.

One finding regarding the Kubernetes issue: in a simple plain docker image both Environment.MachineName and Dns.GetHostName() work and return the same value - have not tried it in Kubernetes.

[gregkalapos]

Done in #1509.