Feature #79 sudo users

Another go at #79

It introduces a small cookbook "sysadmins" that takes sysadmins from the node json file and creates these on the server.

Please have a look and tell me what you think. If you like, I will add a commit that adds this cookbook to the base role and bump the version.

I could not find any tests in the main repo, but maybe I am looking wrong? Am I supposed to run/add tests? And if so, where?

Is it wise to specify a password for users? I suggest we only add a key with a disabled password. If someone wants to enable their password, they can do so after logging in with the given ssh key. What do you think?