Implement `hostlist` operator for RFC 31 constraints

[grondo]

A hostlist operator for RFC 31 job constraints is required to force inclusion or exclusion of nodes by hostname. E.g.:

{ "hostlist" : [ "fluke[16-17]" ] }

would force the inclusion of the listed nodes, while hosts could be excluded by

{ "not": [{ "hostlist": [ "fluke[16-17]" ] }] }