RestAPI: "add new user" by username and password

[mcicolella]

Now we already have an API to add a new user but it requires the following schema as parameter

{ "name": "", "password": "", "roles": [ "" ], "properties": { "values": "java.util.Collection<V>", "empty": false, "entrySet": "Set[java.util.Map.java.util.Map$Entry<K, V>]", "keySet": [ "K" ] } }

We should add another API endpoint in which only username and password (as text parameters) are required. We could add a "role" parameter or set a default one (e.g. guest).
It could be used for #262

[mcicolella]

@p3trur0 what do you think?

[ubaldop]

@mcicolella I'll do that too. Please assign it to me.