Add a dynamic loading of Actions handlers in the Watcher Applier

In watcher, an audit generates a set of actions which
aims at achieving a given goal (lower energy consumption, ...).
It is possible to configure different strategies in order to achieve
each goal. Each strategy is written as a Python class which produces
a set of actions. Today, the set of possible actions is fixed for a
given version of Watcher and enables optimization algorithms to
include actions such as instance migration, changing hypervisor state,
changing power state (ACPI level, ...).

The objective of this patchset is to give the ability to load
the actions dynamically in order to apply the Action Plan.

DocImpact
Partially implements: blueprint watcher-add-actions-via-conf

Change-Id: Idf295b94dca549ac65d4636e8889c8ab2ecc0df6

setup.cfg

@@ -46,6 +46,11 @@ watcher_strategies =
     basic = watcher.decision_engine.strategy.strategies.basic_consolidation:BasicConsolidation
     outlet_temp_control = watcher.decision_engine.strategy.strategies.outlet_temp_control:OutletTempControl
 
+watcher_actions =
+    migrate = watcher.applier.primitives.migration:Migrate
+    nop = watcher.applier.primitives.nop:Nop
+    change_nova_service_state = watcher.applier.primitives.change_nova_service_state:ChangeNovaServiceState
+
 watcher_planners =
     default  = watcher.decision_engine.planner.default:DefaultPlanner