Performance optimization in resolving host patterns

Avoid resolving a pattern that is a plain host. When matching a hostname in the
hosts_cache, just use the host object from there.

When running a task on say 750 hosts, this yields a huge improvement.

lib/ansible/inventory/__init__.py

@@ -213,6 +213,10 @@ class Inventory(object):
         hosts = []
 
         for p in patterns:
+            # avoid resolving a pattern that is a plain host
+            if p in self._hosts_cache:
+                hosts.append(self.get_host(p))
+            else:
                 that = self.__get_hosts(p)
                 if p.startswith("!"):
                     hosts = [ h for h in hosts if h not in that ]
@@ -221,7 +225,6 @@ class Inventory(object):
                 else:
                     to_append = [ h for h in that if h.name not in [ y.name for y in hosts ] ]
                     hosts.extend(to_append)
-        
         return hosts
 
     def __get_hosts(self, pattern):