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?

`<% Array(@ssh_keys).each do |key| %>`

This way the node.json can provide either a string: one ssh-key, an array of ssh-keys or no ssh-keys. It is taken from the often used users::sysadmin cookbook.
I could simplify it, if you wish and simply require a user to always provide an array of ssh-keys.