rfc25: add optional exclusive key to node resource vertex

Problem: Jobspec V1 does not define an exclusive key for resources
in the resource section. This makes it impossible to offer a consistent
interface for users for the `flux mini` commands when only the `-N`
option is used.

Add `exclusive` key to Jobspec V1 only for `node` type resource vertices.
The intent is to ensure that a user can request exclusive access to nodes
whether an instance allocates nodes exclusively by default or not.

Author: grondo

spec_25.rst

@@ -93,7 +93,11 @@ A resource vertex SHALL contain only the following keys:
 
 -  label
 
-The definitions of ``unit``, ``with``, and ``label`` SHALL match
+a ``node`` type resource vertex MAY also contain the following optional keys:
+
+- exclusive
+
+The definitions of ``unit``, ``with``, ``exclusive`` and ``label`` SHALL match
 those found in RFC14. The others are redefined and simplified to mean the
 following: