Loads the network device details if not cached already.

def _retrieve_device_cache(proxy=None):
    '''
    Loads the network device details if not cached already.
    '''
    global DEVICE_CACHE
    if not DEVICE_CACHE:
        if proxy and salt.utils.napalm.is_proxy(__opts__):
            # if proxy var passed and is NAPALM-type proxy minion
            if 'napalm.get_device' in proxy:
                DEVICE_CACHE = proxy['napalm.get_device']()
        elif not proxy and salt.utils.napalm.is_minion(__opts__):
            # if proxy var not passed and is running in a straight minion
            DEVICE_CACHE = salt.utils.napalm.get_device(__opts__)
    return DEVICE_CACHE