add docker based beaker nodesets

Copied from puppet-jenkins

spec/acceptance/nodesets/centos-6-docker.yml

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+HOSTS:
+  centos-6-x64:
+    platform: el-6-x86_64
+    hypervisor : docker
+    image: centos:6
+    docker_preserve_image: true
+    docker_cmd: '["/sbin/init"]'
+    docker_image_commands:
+      - 'yum install -y crontabs tar wget'
+CONFIG:
+  type: foss
+  log_level: debug

spec/acceptance/nodesets/centos-7-docker.Dockerfile

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+FROM centos:7
+MAINTAINER puppet-jenkins
+ENV container docker
+
+# beaker default behavior
+RUN yum clean all
+RUN yum install -y sudo openssh-server openssh-clients curl ntpdate
+RUN ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
+RUN ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
+RUN mkdir -p /var/run/sshd
+RUN echo root:root | chpasswd
+RUN sed -ri 's/^#?PermitRootLogin .*/PermitRootLogin yes/' /etc/ssh/sshd_config
+RUN sed -ri 's/^#?PasswordAuthentication .*/PasswordAuthentication yes/' /etc/ssh/sshd_config
+RUN yum install -y crontabs tar wget
+EXPOSE 22
+
+# based on https://github.com/slafs/dockerfiles/blob/master/centos7-systemd/Dockerfile
+RUN yum -y swap -- remove fakesystemd -- install systemd systemd-libs
+RUN systemctl mask dev-mqueue.mount dev-hugepages.mount \
+    systemd-remount-fs.service sys-kernel-config.mount \
+    sys-kernel-debug.mount sys-fs-fuse-connections.mount \
+    display-manager.service systemd-logind.service
+RUN systemctl disable graphical.target; systemctl enable multi-user.target
+RUN systemctl enable sshd.service
+VOLUME ["/sys/fs/cgroup"]
+
+# provides /usr/sbin/service required by the service redhat provider
+RUN yum install -y initscripts
+
+CMD  ["/usr/lib/systemd/systemd"]

spec/acceptance/nodesets/centos-7-docker.yml

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+HOSTS:
+  centos-7-x64:
+    platform: el-7-x86_64
+    hypervisor : docker
+    docker_preserve_image: true
+    dockerfile: spec/acceptance/nodesets/centos-7-docker.Dockerfile
+CONFIG:
+  type: foss
+  log_level: debug

spec/acceptance/nodesets/ubuntu-14.04-docker.yml

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+HOSTS:
+  ubuntu-14.04-x64:
+    platform: ubuntu-14.04-amd64
+    hypervisor : docker
+    #image: ubuntu:14.04
+    image: electrical/ubuntu:14.04
+    docker_preserve_image: true
+    docker_cmd: '["/sbin/init"]'
+CONFIG:
+  type: foss
+  log_level: debug