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

watcher/conf/decision_engine.py

@@ -55,8 +55,20 @@ WATCHER_DECISION_ENGINE_OPTS = [
     cfg.IntOpt('check_periodic_interval',
                default=30 * 60,
                mutable=True,
-               help='Interval (in seconds) for checking action plan expiry.')
-]
+               help='Interval (in seconds) for checking action plan expiry.'),
+    cfg.StrOpt('metric_map_path',
+               default='/etc/watcher/metric_map.yaml',
+               help='Path to metric map yaml formatted file. '
+                    ' '
+                    '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. '
+                    'For example:: \n\n'
+                    '  monasca:\n'
+                    '    instance_cpu_usage: VM_CPU\n'
+                    '  gnocchi:\n'
+                    '    instance_cpu_usage: cpu_vm_util\n\n'
+                    'This file is optional.')]
 
 WATCHER_CONTINUOUS_OPTS = [
     cfg.IntOpt('continuous_audit_interval',