Add helper method `os` to get OS info easily

With this helper method, you can write OS specific code like this.

```ruby
if ['rhel', 'centos'].include?(os[:family])
  # rhel related environment spec
  if os[:arch] == 'i386'
    # 32bit environment spec
  else
  end
elsif ['debian', 'ubuntu'].include?(os[:family])
  # debian related environment spec
end
```

Another example with serverspec.

```ruby
describe file('/usr/lib64'), :if os[:arch] == 'x86_64' do
  # ...
end
```

See #110 also.