##### SUMMARY
The current implementation of CachePluginAdjudicator is unable to flush the cache without loading everything in memory using `load_whole_cache`. Iterating over the plugins cache keys would avoid that step.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
ansible.plugins.cache
CachePluginAdjudicator

##### ANSIBLE VERSION
```
ansible 2.9.6
  config file = /home/harm/dev/closso-ansible/ansible.cfg
  configured module search path = ['/home/harm/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/harm/.virtualenvs/ansible/lib/python3.7/site-packages/ansible
  executable location = /home/harm/.virtualenvs/ansible/bin/ansible
  python version = 3.7.3 (default, Apr  3 2019, 05:39:12) [GCC 8.3.0]
```

##### CONFIGURATION
```
CACHE_PLUGIN(/home/harm/dev/closso-ansible/ansible.cfg) = jsonfile
CACHE_PLUGIN_CONNECTION(/home/harm/dev/closso-ansible/ansible.cfg) = ./cache/fact/
CACHE_PLUGIN_TIMEOUT(/home/harm/dev/closso-ansible/ansible.cfg) = 86400
DEFAULT_CALLBACK_PLUGIN_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/plugins/callback', '/usr/share/ansible/plugins/callback']
DEFAULT_CALLBACK_WHITELIST(/home/harm/dev/closso-ansible/ansible.cfg) = ['profile_tasks']
DEFAULT_FILTER_PLUGIN_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/plugins/filter', '/usr/share/ansible/plugins/filter']
DEFAULT_GATHERING(/home/harm/dev/closso-ansible/ansible.cfg) = smart
DEFAULT_HOST_LIST(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/netbox_inventory.yml']
DEFAULT_LOOKUP_PLUGIN_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/plugins/lookup', '/usr/share/ansible/plugins/lookup']
DEFAULT_ROLES_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/roles']
DEFAULT_VARS_PLUGIN_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/plugins/vars', '/usr/share/ansible/plugins/vars']
DOC_FRAGMENT_PLUGIN_PATH(/home/harm/dev/closso-ansible/ansible.cfg) = ['/home/harm/dev/closso-ansible/plugins/doc_fragments', '/usr/share/ansible/plugins/doc_fragments']
HOST_KEY_CHECKING(/home/harm/dev/closso-ansible/ansible.cfg) = False
INVENTORY_CACHE_ENABLED(/home/harm/dev/closso-ansible/ansible.cfg) = True
INVENTORY_ENABLED(/home/harm/dev/closso-ansible/ansible.cfg) = ['host_list', 'script', 'yaml', 'ini', 'netbox']
TRANSFORM_INVALID_GROUP_CHARS(/home/harm/dev/closso-ansible/ansible.cfg) = never
USE_PERSISTENT_CONNECTIONS(/home/harm/dev/closso-ansible/ansible.cfg) = True
```

##### STEPS TO REPRODUCE

```
>>> cache = CachePluginAdjudicator('jsonfile', _uri='./cache/netbox')
>>> cache._plugin.keys()
['netbox_b42bfs_d14a8', 'netbox_b42bfs_81f3c']
>>> cache.keys()
dict_keys([])
>>> cache.flush()
>>> cache._plugin.keys()
['netbox_b42bfs_d14a8', 'netbox_b42bfs_81f3c']
>>> cache.keys()
dict_keys([])
>>> cache.load_whole_cache()
>>> cache.keys()
dict_keys(['netbox_b42bfs_d14a8', 'netbox_b42bfs_81f3c'])
>>> cache.flush()
>>> cache.keys()
dict_keys([])
>>> cache._plugin.keys()
[]
```

##### EXPECTED RESULTS

Cache should be flushed without loading everything in memory.


##### ACTUAL RESULTS

`cache.flush()` does nothing.

"""
    This is for backwards compatibility.  Will be removed after deprecation.  It was removed as it
    wasn't actually part of the cache plugin API.  It's actually the code to make use of cache
    plugins, not the cache plugin itself.  Subclassing it wouldn't yield a usable Cache Plugin and
    there was no facility to use it as anything else.
    """
    def __init__(self, *args, **kwargs):
        display.deprecated('ansible.plugins.cache.FactCache has been moved to'
                           ' ansible.vars.fact_cache.FactCache.  If you are looking for the class'
                           ' to subclass for a cache plugin, you want'
                           ' ansible.plugins.cache.BaseCacheModule or one of its subclasses.',
                           version='2.12', collection_name='ansible.builtin')
        super(FactCache, self).__init__(*args, **kwargs)
class BaseCacheModule(AnsiblePlugin):