Add support for SSH Host Key Checking

By default it seems that SSH host key checking has been disabled. This
patch makes it optional. If a variable named known_hosts is passed in,
the key checking will be enabled. The variable should contain the
complete contents of the known_hosts file, which must contain the public
key(s) of the host(s) in the inventory.

.github/workflows/test.yml

@@ -65,11 +65,15 @@ jobs:
             Subsystem sftp /usr/lib/openssh/sftp-server
           EOF
           sudo systemctl restart sshd
+          echo 'SSH_KNOWN_HOSTS<<EOF' >> $GITHUB_ENV
+          echo $(ssh-keyscan localhost) >> $GITHUB_ENV
+          echo 'EOF' >> $GITHUB_ENV
       - name: With everything
         uses: ./
         with:
           playbook: playbook.yml
           key: ${{env.SSH_PRIVATE_KEY}}
+          known_hosts: ${{env.SSH_KNOWN_HOSTS}}
           directory: test
           vault_password: test
           requirements: requirements.yml