user module: return public SSH key

Return public SSH key if the user module is called with generate_ssh_key=yes.
Since "user" doesn't overwrite files, this also allows querying of existing
public keys.

Used in playbooks together with the "register" keyword, the returned key can be
passed to the "authorized_key" module allowing easy setup of SSH public key
authentication between remote hosts.
Merge pull request #2673 from b6d/user-module-extension

user module: return public SSH key