flux-resource: add queue argument to ResourceSetExtra initializer

Problem: An empty resource set cannot have any associated properties
and therefore cannot be in a queue. However, in `flux resource list`
output, it may be useful to display an "empty" line of output for one
or more queues. For instance, when no nodes are allocated, down, etc.

Allow the ResourceSetExtra class to have a forced queue associated
with it. If set in the initializer, then instead of iterating
properties to deterimine the resource set queue the manually set
queue is simply returned. This allows an empty resource set to have
an associated queue.

Author: grondo

src/cmd/flux-resource.py

@@ -492,8 +492,9 @@ def drain_list(args):
 
 
 class ResourceSetExtra(ResourceSet):
-    def __init__(self, arg=None, version=1, flux_config=None):
+    def __init__(self, arg=None, version=1, flux_config=None, queue=None):
         self.flux_config = flux_config
+        self._queue = queue
         if isinstance(arg, ResourceSet):
             self._rset = arg
             if arg.state:
@@ -517,6 +518,13 @@ def propertiesx(self):
 
     @property
     def queue(self):
+        #  Note: queue may be set manually in self._queue for an empty
+        #  ResourceSet, which cannot otherwise have an associated queue.
+        if self._queue is not None:
+            return self._queue
+
+        #  If self._queue is not set, then build list of queues from
+        #  set properties and queue configuration:
         queues = ""
         if self.flux_config and "queues" in self.flux_config:
             if not self.ranks: