Fix exception handling in _get_host_metrics()

In resource_tracker.py, the exception path of _get_host_metrics()
misspells variable name 'monitor'. When exception occurs, this
misspelling prevents us to log warning message. This code change
corrects the misspelling.

Closes-Bug: 1394052

Conflicts:
        nova/tests/unit/compute/test_resource_tracker.py

Change-Id: I8a99a4ceb53c89038e8b292a15ad9fd20daa4233
(cherry picked from commit e09880dcd0f7ae766cae170c5b30fbabbe85eac3)
Merge "Fix exception handling in _get_host_metrics()" into stable/juno