Allow for global datasources preference from config

Allows to define a global preference for metric datasources with the
ability for strategy specific overrides. In addition, strategies which
do not require datasources have the config options removed this is
done to prevent confusion.

Some documentation that details the inner workings of selecting
datasources is updated.

Imports for some files in watcher/common have been changed to resolve
circular dependencies and now match the overall method to import
configuration.

Addtional datasources will be retrieved by the manager if the
datasource throws an error.

Implements: blueprint global-datasource-preference
Change-Id: I6fc455b288e338c20d2c4cfec5a0c95350bebc36

doc/source/contributor/plugin/strategy-plugin.rst

@@ -285,8 +285,15 @@ The following code snippet shows how datasource_backend is defined:
     @property
     def datasource_backend(self):
         if not self._datasource_backend:
+
+            # Load the global preferred datasources order but override it
+            # if the strategy has a specific datasources config
+            datasources = CONF.watcher_datasources
+            if self.config.datasources:
+                datasources = self.config
+
             self._datasource_backend = ds_manager.DataSourceManager(
-                config=self.config,
+                config=datasources,
                 osc=self.osc
             ).get_backend(self.DATASOURCE_METRICS)
         return self._datasource_backend