Use beaker-hostgenerator for docker acceptance tests

This also removes sudo: required and dist: trusty since that's implied
by Travis when using docker.

config_defaults.yml

@@ -4,10 +4,8 @@
   docker_defaults:
     # values will replace @@SET@@ with the docker_sets' value
     rvm: 2.4.3
-    sudo: required
-    dist: trusty
     services: docker
-    env: PUPPET_INSTALL_TYPE=agent BEAKER_debug=true BEAKER_set=@@SET@@ CHECK=beaker
+    env: PUPPET_INSTALL_TYPE=agent BEAKER_debug=true BEAKER_setfile=@@SET@@{hypervisor=docker} CHECK=beaker
     bundler_args: --without development release
   includes:
   - rvm: 2.1.9
@@ -69,6 +67,8 @@ Gemfile:
       - gem: winrm
       - gem: beaker-rspec
       - gem: serverspec
+      - gem: beaker-hostgenerator
+        version: '>= 1.1.10'
       - gem: beaker-puppet_install_helper
       - gem: beaker-module_install_helper
         require: false