Rudder

In 2.10 we can manage the skipidentify option from the webapp (which is awesome)

However there is still code related to skipidentify and android:
https://github.com/Normation/rudder-techniques/blob/branches/rudder/2.10/initial-promises/node-server/failsafe.cf#L36
https://github.com/Normation/rudder-techniques/blob/branches/rudder/2.10/techniques/system/common/1.0/failsafe.st#L39

Mobile should not be treated differently (we can have unix devices without dns, and why not mobile device with dns)

The PR try to manage every device the same way.

Please review, I must say that I am not confident in my understanding of failsafe.{cf,st} vs promises.{cf,st}