Fetch a key value.

def fetch(bank, key):
    '''
    Fetch a key value.
    '''
    c_key = '{0}/{1}'.format(bank, key)
    try:
        _, value = api.kv.get(c_key)
        if value is None:
            return {}
        return __context__['serial'].loads(value['Value'])
    except Exception as exc:
        raise SaltCacheError(
            'There was an error reading the key, {0}: {1}'.format(
                c_key, exc
            )
        )