Allow negative node usage declaration.

For example, "-xdcr" means don't allow xdcr operations to run on that
node.  By default, all operations are allowed.  Then, negative
declarations ("-n1ql,-moxi") take precedence.  If every declaration is
negative, then all operations are assumed to be allowed.  If one or
more positive declarations are provided, then all operations are
assumed to be disallowed.  For example, "bucket,moxi" means only allow
bucket and moxi operations.