Fix _build_instance_node for building Compute CDM

As of Nova API microversion 2.47, response of GET /servers/detail has flavor
which contains a subset of the actual flavor information used to create the
server instance, represented as a nested dictionary.

Since current watcher nova default api version is 2.53(Pike), This patch
follows the API response change.

Change-Id: Ia575950f0702afa1d093f03ca8ddedd3c410b7de
Closes-Bug: #1722462

watcher/common/nova_helper.py

@@ -70,9 +70,6 @@ class NovaHelper(object):
     def get_service(self, service_id):
         return self.nova.services.find(id=service_id)
 
-    def get_flavor(self, flavor_id):
-        return self.nova.flavors.get(flavor_id)
-
     def get_aggregate_list(self):
         return self.nova.aggregates.list()