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
b620081714