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/tests/common/test_nova_helper.py

@@ -123,7 +123,6 @@ class TestNovaHelper(base.TestCase):
         hypervisor = self.fake_hypervisor(hypervisor_id, hypervisor_name)
         self.fake_nova_hypervisor_list(
             nova_util,
-            fake_find=hypervisor,
             fake_list=[hypervisor])
         nova_util.nova.hypervisors.search.return_value = [hypervisor]
         # verify that the compute node can be obtained normally by name