Get grains for proxy minion

    .. code-block: bash

        salt '*' onyx.cmd grains

def grains():
    '''
    Get grains for proxy minion

    .. code-block: bash

        salt '*' onyx.cmd grains
    '''
    if not DETAILS['grains_cache']:
        ret = system_info()
        log.debug(ret)
        DETAILS['grains_cache'].update(ret)
    return {'onyx': DETAILS['grains_cache']}