Fix docstring for GET /os-migrations and related DB API

The GET /os-migrations API and migration_get_all_by_filters
DB API method are not actually filtering migrations by status. The API
will only do that if the user requests the migrations to be filtered by
status using query paramters to the REST API.

The docstrings related to this were confusing since the
GET /servers/{server_id}/migrations REST API *will* filter by
in-progress status.

Change-Id: Ie9c500ae62c23930bdb8f5700dbdb9f149105e9a

nova/api/openstack/compute/migrations.py

@@ -69,7 +69,7 @@ class MigrationsController(wsgi.Controller):
 
     @extensions.expected_errors(())
     def index(self, req):
-        """Return all migrations in progress."""
+        """Return all migrations using the query parameters as filters."""
         context = req.environ['nova.context']
         context.can(migrations_policies.POLICY_ROOT % 'index')
         migrations = self.compute_api.get_migrations(context, req.GET)

nova/db/api.py

@@ -564,7 +564,7 @@ def migration_get_in_progress_by_host_and_node(context, host, node):
 
 
 def migration_get_all_by_filters(context, filters):
-    """Finds all migrations in progress."""
+    """Finds all migrations using the provided filters."""
     return IMPL.migration_get_all_by_filters(context, filters)