Strategy plugins documentation

Following https://review.openstack.org/#/c/245903, this patchset
documents the implementation and integration of custom Strategies
with Watcher.

Change-Id: I573758695d67d523834f145824b6f6cfdd8997fe

watcher/decision_engine/api/strategy/strategy.py

@@ -27,6 +27,10 @@ LOG = log.getLogger(__name__)
 
 @six.add_metaclass(abc.ABCMeta)
 class BaseStrategy(object):
+    """A Strategy is an algorithm implementation which is able to find a
+    Solution for a given Goal.
+    """
+
     def __init__(self, name=None, description=None):
         self._name = name
         self.description = description