Allow using file to override metric map

Override the metric map of each datasource as soon as it is created by
the manager. This override comes from a file whose path is provided by
a setting in config file.

Loading at creation time allows the correct datasource be used when
get_backend is called, this allows loading a datasource whose metric
names get updated outside the watcher's codebase.

The function 'load_metric_map' returns empty-dict in any error case.
Also in case the file is empty where safe_load is unable finds any
yaml documents, it will return None. [1]

Some minor refactoring in the test_manager file for readability and
added tests for file load and metric override.

1 - https://pyyaml.org/wiki/PyYAMLDocumentation

Change-Id: I1df16245f4c7dfd34066f3ab0553cd67154faa58
Implements: blueprint file-based-metric-map

releasenotes/notes/file-based-metric-map-c2af62b5067895df.yaml

@@ -0,0 +1,11 @@
+---
+features:
+  - |
+    Allow using file to override metric map. Override the metric map of
+    each datasource as soon as it is created by the manager. This override
+    comes from a file whose path is provided by a setting in config file.
+    The setting is `watcher_decision_engine/metric_map_path`.  The file
+    contains a map per datasource whose keys are the metric names as
+    recognized by watcher and the value is the real name of the metric
+    in the datasource. This setting defaults to `/etc/watcher/metric_map.yaml`,
+    and presence of this file is optional.