Implement basic `sh' task action feature:

* Executes a command on the remote host;
* Forwards remote standard output.

spec/producer/core/actions/shell_command_spec.rb

@@ -0,0 +1,27 @@
+require 'spec_helper'
+
+module Producer::Core
+  describe Actions::ShellCommand do
+    let(:env)           { Env.new }
+    let(:command_args)  { 'hello from remote host'}
+    let(:command)       { "echo #{command_args}" }
+    subject(:sh)        { Actions::ShellCommand.new(env, command) }
+
+    describe '#apply' do
+      before do
+        env.output = StringIO.new
+      end
+
+      it 'delegates the call to env.remote.execute method' do
+        expect(env.remote).to receive(:execute).with(command)
+        sh.apply
+      end
+
+      it 'forwards the returned output to env.output' do
+        allow(env.remote).to receive(:execute) { command_args }
+        expect(env).to receive(:output).with(command_args)
+        sh.apply
+      end
+    end
+  end
+end