Implement serverspec elements

serverspec aims to run service validation during TripleO/Puppet
deployment by using step-by-step workflow.

Example: step 1 contains MySQL Galera cluster, so Puppet will take care
of configuration and serverspec will ensure MySQL is actually up and
running, otherwise we consider the stack failed.

This patchs:
* Installs serverspec by using Gem (source).
  For now, there is no package for any distribution, so until we have one,
  let's use source method by running `gem install`, that will take care of
  serverspec and dependencies.
* Installs serverspec-tests from rdo-management.
  For now, serverspec-tests are hosted on github/rdo-management, so
  until we have packaging, we use source install method.

Change-Id: I8efc9a2278e972fe0c61e12823a9250a4a1bc27d

elements/serverspec/README.md

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+Install serverspec.
+
+Supports gem based for now. Package mode will be eventually supported later.
+
+To use gem:
+
+ export DIB\_INSTALLTYPE\_serverspec=source
+
+Configuration
+-------------
+None.

elements/serverspec/element-deps

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+package-installs
+pkg-map

elements/serverspec/install.d/package-installs-serverspec

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+rubygems_package

elements/serverspec/install.d/serverspec-source-install/76-serverspec-source

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+#!/bin/bash
+set -eux
+
+gem install serverspec

elements/serverspec/pkg-map

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+{
+  "default": {
+    "rubygems_package": "rubygems"
+  }
+}