Added information on plugin mechanism to glossary

Added extra information regarding the plugin mechanism for:
action, strategy, and Watcher planner.

Change-Id: I9a7523282e229b83c16b06e3806ff795a0699c78
Closes-Bug: #1558470

watcher/api/controllers/v1/action.py

@@ -49,6 +49,10 @@ be one of the following:
 -  **CANCELLED** : the :ref:`Action <action_definition>` was in **PENDING** or
    **ONGOING** state and was cancelled by the
    :ref:`Administrator <administrator_definition>`
+
+:ref:`Some default implementations are provided <watcher_planners>`, but it is
+possible to :ref:`develop new implementations <implement_action_plugin>` which
+are dynamically loaded by Watcher at launch time.
 """
 
 import datetime

watcher/decision_engine/planner/base.py

@@ -37,6 +37,10 @@ congestion which may decrease the :ref:`SLA <sla_definition>` for
 It is also important to schedule :ref:`Actions <action_definition>` in order to
 avoid security issues such as denial of service on core OpenStack services.
 
+:ref:`Some default implementations are provided <watcher_planners>`, but it is
+possible to :ref:`develop new implementations <implement_planner_plugin>`
+which are dynamically loaded by Watcher at launch time.
+
 See :doc:`../architecture` for more details on this component.
 """
 

watcher/decision_engine/strategy/strategies/base.py

@@ -30,6 +30,10 @@ to find an optimal :ref:`Solution <solution_definition>`.
 When a new :ref:`Goal <goal_definition>` is added to the Watcher configuration,
 at least one default associated :ref:`Strategy <strategy_definition>` should be
 provided as well.
+
+:ref:`Some default implementations are provided <watcher_strategies>`, but it
+is possible to :ref:`develop new implementations <implement_strategy_plugin>`
+which are dynamically loaded by Watcher at launch time.
 """
 
 import abc