ci: Use dedicated slave instead of Docker

Using a dedicated slave node instead of a Docker container has a few
advantages:

* Persistent configuration is possible, without making weird assumptions
  (e.g. Jenkins UID/GID) in the Dockerfile
* Can limit concurrent deployments by controlling executor count on the
  node

ci/Dockerfile

@@ -1,14 +0,0 @@
-FROM fedora:27
-
-RUN dnf install -y \
-	ansible \
-	openssh-clients \
-	python2-netaddr
-
-# XXX These UID/GIDs must match the Jenkins UID/GID on the agent
-RUN groupadd -g 992 jenkins
-RUN useradd -u 995 -g jenkins -c Jenkins -d /var/lib/jenkins -m jenkins
-
-RUN install -m 700 -o jenkins -g jenkins -d /var/lib/jenkins/.ssh
-COPY known_hosts /var/lib/jenkins/.ssh/
-RUN chown jenkins:jenkins /var/lib/jenkins/.ssh/known_hosts