Move proxy_command to config and support templating (#1226)

Migrate the proxy_command specification to the config and add support
for user override template to be used for edge cases. Moving it to the
config allows mistakes in the interpolation to be caught before the
machine is brought up.

Note this uses a more restrictive replacement to avoid requiring
escaping of '%' or '$'.

Issue #921 already partially resolved thanks to @ElArtista, this
completes the fix by allowing users to override as needed.

Fixes: #921

spec/unit/config_spec.rb

@@ -335,6 +335,101 @@ describe VagrantPlugins::ProviderLibvirt::Config do
         end
       end
     end
+
+    context '@proxy_command' do
+      before(:example) do
+        stub_const("ENV", fake_env)
+        fake_env['HOME'] = "/home/tests"
+      end
+
+      [
+        # no connect_via_ssh
+        [
+          {:host => "remote"},
+          nil,
+        ],
+
+        # connect_via_ssh
+        [ # host
+          {:connect_via_ssh => true, :host => 'remote'},
+          "ssh 'remote' -W %h:%p",
+        ],
+        [ # include user
+          {:connect_via_ssh => true, :host => 'remote', :username => 'myuser'},
+          "ssh 'remote' -l 'myuser' -W %h:%p",
+        ],
+        [ # include user and default ssh key exists
+          {:connect_via_ssh => true, :host => 'remote', :username => 'myuser'},
+          "ssh 'remote' -l 'myuser' -i '/home/tests/.ssh/id_rsa' -W %h:%p",
+          {
+            :setup => ContextualProc.new {
+              expect(File).to receive(:file?).with("/home/tests/.ssh/id_rsa").and_return(true)
+            }
+          }
+        ],
+
+        # disable id_ssh_key_file
+        [
+          {:connect_via_ssh => true, :host => 'remote', :id_ssh_key_file => nil},
+          "ssh 'remote' -W %h:%p",
+        ],
+        [ # include user
+          {:connect_via_ssh => true, :host => 'remote', :id_ssh_key_file => nil},
+          "ssh 'remote' -W %h:%p",
+        ],
+
+        # use @uri
+        [
+          {:uri => 'qemu+ssh://remote/system'},
+          "ssh 'remote' -W %h:%p",
+        ],
+        [
+          {:uri => 'qemu+ssh://myuser@remote/system'},
+          "ssh 'remote' -l 'myuser' -W %h:%p",
+        ],
+        [
+          {:uri => 'qemu+ssh://remote/system?keyfile=/some/path/to/keyfile'},
+          "ssh 'remote' -i '/some/path/to/keyfile' -W %h:%p",
+          {:allow_failure => "keyfile not yet inferred from uri"},
+        ],
+
+        # provide custom template
+        [
+          {:connect_via_ssh => true, :host => 'remote', :proxy_command => "ssh {host} nc %h %p" },
+          "ssh remote nc %h %p",
+        ],
+        [
+          {:connect_via_ssh => true, :host => 'remote', :username => 'myuser', :proxy_command => "ssh {host} nc %h %p" },
+          "ssh remote nc %h %p",
+        ],
+        [
+          {:connect_via_ssh => true, :host => 'remote', :username => 'myuser', :proxy_command => "ssh {host} -l {username} nc %h %p" },
+          "ssh remote -l myuser nc %h %p",
+        ],
+      ].each do |inputs, proxy_command, options|
+        opts = {}
+        opts.merge!(options) if options
+
+        it "should handle inputs #{inputs}" do
+          # allow some of these to fail for now if marked as such
+          if !opts[:allow_failure].nil?
+            pending(opts[:allow_failure])
+          end
+
+          if !opts[:setup].nil?
+            opts[:setup].apply(binding)
+          end
+
+          inputs.each do |k, v|
+            subject.instance_variable_set("@#{k}", v)
+          end
+
+          subject.finalize!
+
+          expect(subject.proxy_command).to eq(proxy_command)
+        end
+      end
+    end
   end
 
   def assert_invalid