Add type spec for process_post.
Covered all the functions within admin_nodes.erl.

Some of the security related definitions are too permissive and really need to
be locked down better. Unfortunately at this time I don't understand well
enough how they work to do that.
Use boolean() type rather than true|false.
Cover all functions within admin_cluster_down with specs.

Again some of the security related functions are too permissive.