Add role for adding ssh key to remote nodes

Some tasks, such as proposal tasks, need to make outbound ssh
connections via non-ansible means. Add a role that takes a secret and
writes out a private key and optionally a known hosts entry.

Change-Id: I569b93ba0e775896b5a4f59b544fe32d1e2dce9b

roles/add-sshkey/README.rst

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+Add an ssh key to the host so that non-ansible ssh connections can be made.
+
+**Role Variables**
+
+.. zuul:rolevar:: ssh_key
+
+  Complex argument which contains the ssh key information. It is
+  expected that this argument comes from a `Secret`.
+
+  .. zuul:rolevar:: ssh_known_hosts
+
+    String containing known host signature for the remote host.
+
+  .. zuul:rolevar:: ssh_private_key
+
+    Contents of the ssh private key to use.
+
+  .. zuul:rolevar:: fqdn
+
+    The FQDN of the remote host.

roles/add-sshkey/tasks/main.yaml

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+- name: Write out ssh key
+  file:
+    path: "~/.ssh/id_rsa"
+    content: "{{ ssh_key.ssh_private_key }}"
+
+- name: Add host key information to known hosts
+  known_hosts:
+    name: "{{ ssh_key.fqdn }}"
+    key: "{{ ssh_key.ssh_known_hosts }}"
+  when: ssh_key.ssh_known_hosts is defined and ssh_key.fqdn is defined