Add a dynamic loading of the Watcher Planner implementation

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 extend the
default set of planner algorithms currently available in Watcher
using Stevedore.

The doc need to explain how create a new planner.

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

Change-Id: I2fd73f8c4a457ee391d764a7a3f494deecd2634f

setup.cfg

@@ -46,6 +46,9 @@ watcher_strategies =
     basic = watcher.decision_engine.strategy.strategies.basic_consolidation:BasicConsolidation
     outlet_temp_control = watcher.decision_engine.strategy.strategies.outlet_temp_control:OutletTempControl
 
+watcher_planners =
+    default  = watcher.decision_engine.planner.default:DefaultPlanner
+
 [build_sphinx]
 source-dir = doc/source
 build-dir = doc/build