Fix: Allow usage of net=host with docker executor

If you try to set network_mode = host in the config for docker executor, you will get an error about conflicting options hostname and network mode:

level=error msg="Handler for POST /v1.18/containers/create returned error: Conflicting options: hostname and the network mode"

Sample config:

concurrent = 1
check_interval = 0

  name = "artem-test-runner"
  url = "<--URL-->"
  token = "<--TOKEN-->"
  executor = "docker"
    tls_verify = false
    image = "ruby:2.1"
    privileged = false
    disable_cache = false
    volumes = ["/cache"]
    network_mode = "host"

Closes: gitlab-org/gitlab-ci-multi-runner#903