Cluster data model collector plugin documentation

In this changeset, I wrote down the documentation on how to implement
a cluster data model collector plugin for Watcher.

This documentation corresponds to part 1 of the associated
specification.

Change-Id: Iac72b933df95252163033cd559d13348075a9b16
Partially-Implements: blueprint cluster-model-objects-wrapper

watcher/decision_engine/model/mapping.py

@@ -30,7 +30,7 @@ class Mapping(object):
         self.lock = threading.Lock()
 
     def map(self, hypervisor, vm):
-        """Select the hypervisor where the instance are launched
+        """Select the hypervisor where the instance is launched
 
         :param hypervisor: the hypervisor
         :param vm: the virtual machine or instance