##### SUMMARY
add_host will return 'changed' for every invocation, it does not matter if the host already existed and no changes were actually made to inventory

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
add_host
##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
2.7, 2.8, 2.9, 2.10
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
all

##### STEPS TO REPRODUCE

Simplest example, but also happens when specifying variables and/or groups

test.yml
```yaml
- hosts: localhost
  gather_facts: false
  tasks:
    - add_host: name=testhost
```
test_inv.ini
```ini
testhost
```
run:
``` ansible-playbook -i test_inv.ini test.yml```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
```
 _________________
< TASK [add_host] >
 -----------------
       \   ,__,
        \  (oo)____
           (__)    )\
              ||--|| *

OK: [localhost]
```

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below
 _________________
< TASK [add_host] >
 -----------------
       \   ,__,
        \  (oo)____
           (__)    )\
              ||--|| *

changed: [localhost]
```

self._flushed_hosts = dict()
        self._results = deque()
        self._handler_results = deque()
        self._results_lock = threading.Condition(threading.Lock())
        
        self._results_thread = threading.Thread(target=results_thread_main, args=(self,))
        self._results_thread.daemon = True
        self._results_thread.start()
        
        
        self._active_connections = dict()
        
        
        
        self._hosts_cache = []
        self._hosts_cache_all = []
        self.debugger_active = C.ENABLE_TASK_DEBUGGER
    def _set_hosts_cache(self, play, refresh=True):
        """Responsible for setting _hosts_cache and _hosts_cache_all
        See comment in ``__init__`` for the purpose of these caches
        """
        if not refresh and all((self._hosts_cache, self._hosts_cache_all)):
            return
        if Templar(None).is_template(play.hosts):
            _pattern = 'all'
        else:
            _pattern = play.hosts or 'all'
        self._hosts_cache_all = [h.name for h in self._inventory.get_hosts(pattern=_pattern, ignore_restrictions=True)]
        self._hosts_cache = [h.name for h in self._inventory.get_hosts(play.hosts, order=play.order)]
    def cleanup(self):