Accept Hetzner API 404 for delete_key as success

If it is gone, there's no need to halt execution.  This can cause CI
runs to fail, in the case where a key was deleted but, for one reason
or another, the deletion success was not recorded.

Perhaps our API should not inflict a similar damage on the consumer,
see
#597 (comment)

lib/hosting/hetzner_apis.rb

@@ -47,7 +47,7 @@ def delete_key(key)
       user: @host.user,
       password: @host.password,
       headers: {"Content-Type" => "application/x-www-form-urlencoded"})
-    connection.delete(path: "/key/#{fingerprint}", expects: 200)
+    connection.delete(path: "/key/#{fingerprint}", expects: [200, 404])
 
     nil
   end

spec/lib/hosting/hetzner_apis_spec.rb

@@ -64,6 +64,12 @@
       Excon.stub({path: "/key/8003339382ac5baa3637f813becce5e4", method: :delete}, {status: 500, body: ""})
       expect { hetzner_apis.delete_key(key_data) }.to raise_error Excon::Error::InternalServerError
     end
+
+    it "regards a missing key as deleted" do
+      key_data = "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDQ8Z9Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0Z0"
+      Excon.stub({path: "/key/8003339382ac5baa3637f813becce5e4", method: :delete}, {status: 404, body: ""})
+      expect { hetzner_apis.delete_key(key_data) }.to be_nil
+    end
   end
 
   describe "get_main_ip4" do