Add support for plugin root level command

Completely generic plugins can be now created:

  @RequiresCapability(
      value = GlobalCapability.ADMINISTRATE_SERVER,
      scope = CapabilityScope.CORE)
  public class ShellCommand extends SshCommand {
    @Override
    protected void parseCommandLine(Object options) throws UnloggedFailure {
      // No op: turn off parsing utility
    }

    @Override
    protected void run() throws UnloggedFailure, IOException {
      String command = getName().substring(getPluginName().length() + 1);
      List<String> c = Lists.newArrayListWithCapacity(getArguments().length + 1);
      c.add(command);
      c.addAll(Arrays.asList(getArguments()));
      ProcessBuilder proc = new ProcessBuilder(c);
      proc.start();
      [...]
    }
  }

SSH module is bound as:

  public class SshModule extends SingleCommandPluginModule {
    @Override
    protected void configure(LinkedBindingBuilder<Command> b) {
      b.to(ShellCommand.class);
    }
  }

If the plugin above is deployed under sh.jar file in $site/plugins
directory, generic commands can be called without specifing the
actually SSH command:

  $ ssh gerrit sh ls -all -R
  $ ssh gerrit sh ps -a -ef

Inspired-by: David Ostrovsky <david@ostrovsky.org>
Change-Id: If0bdce5db82b436723090e17088923c44b5e11eb

gerrit-sshd/src/main/java/com/google/gerrit/sshd/SingleCommandPluginModule.java

@@ -0,0 +1,45 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.sshd;
+
+import com.google.common.base.Preconditions;
+import com.google.gerrit.extensions.annotations.PluginName;
+import com.google.inject.binder.LinkedBindingBuilder;
+
+import org.apache.sshd.server.Command;
+
+import javax.inject.Inject;
+
+/**
+ * Binds one SSH command to the plugin name itself.
+ * <p>
+ * Cannot be combined with {@link PluginCommandModule}.
+ */
+public abstract class SingleCommandPluginModule extends CommandModule {
+  private CommandName command;
+
+  @Inject
+  void setPluginName(@PluginName String name) {
+    this.command = Commands.named(name);
+  }
+
+  @Override
+  protected final void configure() {
+    Preconditions.checkState(command != null, "@PluginName must be provided");
+    configure(bind(Commands.key(command)));
+  }
+
+  protected abstract void configure(LinkedBindingBuilder<Command> bind);
+}