Optimize hypervisor API calls

The nova CDM builder code and notification handling
code had some inefficiencies when it came to looking
up a hypevisor to get details. The general pattern
used before was:

1. get the minimal hypervisor information by hypervisor_hostname
2. make another query to get the hypervisor details by id

In the notifications case, it was actually three calls because
the first is listing hyprvisors to filter client-side by service
host.

This change collapses 1 and 2 above into a single API call
to get the hypervisor by hypervisor_hostname with details
which will include the service (compute) host information
which is what get_compute_node_by_id() was being used for.
Now that nothing is using get_compute_node_by_id it is removed.

There is more work we could do in get_compute_node_by_hostname
if the compute API allowed filtering hypervisors by service
host so a TODO is left for that.

One final thing: the TODO in get_compute_node_by_hostname about
there being more than one hypervisor per compute service host
for vmware vcenter is not accurate - nova's vcenter driver
hasn't supported a host:node 1:M topology like that since the
Liberty release [1]. The only in-tree driver in nova that supports
1:M is the ironic baremetal driver, so the comment is updated.

[1] Ifc17c5049e3ed29c8dd130339207907b00433960

Depends-On: https://review.opendev.org/661785/
Change-Id: I5e0e88d7b2dd1a69117ab03e0e66851c687606da

watcher/decision_engine/model/collector/nova.py

@@ -258,19 +258,15 @@ class ModelBuilder(object):
                 [node.hypervisor_hostname for node in all_nodes])
         LOG.debug("compute nodes: %s", compute_nodes)
         for node_name in compute_nodes:
-            # TODO(mriedem): Change this to list hypervisors with details
-            # so we don't have to call get_compute_node_by_id. It requires
-            # changes to python-novaclient.
             cnode = self.nova_helper.get_compute_node_by_name(node_name,
-                                                              servers=True)
+                                                              servers=True,
+                                                              detailed=True)
             if cnode:
-                # Get the node details (like the service.host).
-                node_info = self.nova_helper.get_compute_node_by_id(
-                    cnode[0].id)
+                node_info = cnode[0]
                 self.add_compute_node(node_info)
                 # node.servers is a list of server objects
                 # New in nova version 2.53
-                instances = getattr(cnode[0], "servers", None)
+                instances = getattr(node_info, "servers", None)
                 self.add_instance_node(node_info, instances)
 
     def add_compute_node(self, node):