Retries for installing neu apt key

Keyservers tend to be offline from time to time and addidtional to iss #7 from hedgehog the apt-key adv should have a certain number of retries and a retry_delay
Somehting like:

``` ruby
execute "install-key #{new_resource.key}" do
   command "apt-key adv --keyserver #{new_resource.keyserver} --recv #{new_resource.key}"
   action :nothing
   retries 3
   retry_delay 5
end.run_action(:run)
```
