Add Matchers for Chefspec

* Add methods:
  - create_deploy_key
  - delete_deploy_key
  - add_deploy_key
  - remove_deploy_key

* Example:

``` 
require 'chefspec'

describe 'example::default' do
  let(:chef_run) { ChefSpec::SoloRunner.converge(described_recipe) }

  it 'deploy key foo' do
    expect(chef_run).to create_deploy_key('foo')
  end
end
```