##### 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]
```

name, port = parse_address(new_name, allow_ranges=False)
        except Exception:
            
            name = new_name
            port = None
        if port:
            self._task.args['ansible_ssh_port'] = port
        groups = self._task.args.get('groupname', self._task.args.get('groups', self._task.args.get('group', '')))
        
        new_groups = []
        if groups:
            if isinstance(groups, list):
                group_list = groups
            elif isinstance(groups, string_types):
                group_list = groups.split(",")
            else:
                raise AnsibleError("Groups must be specified as a list.", obj=self._task)
            for group_name in group_list:
                if group_name not in new_groups:
                    new_groups.append(group_name.strip())
        
        host_vars = dict()
        special_args = frozenset(('name', 'hostname', 'groupname', 'groups'))
        for k in self._task.args.keys():
            if k not in special_args:
                host_vars[k] = self._task.args[k]
        result['changed'] = True
        result['add_host'] = dict(host_name=name, groups=new_groups, host_vars=host_vars)
        return result