Fix response_key parameter usage in BaseManager

Methods _get() _list() and _post() in BaseManager class requires
mandatory parameter `response_key`. This parameter is the key to be
looked up in response dictionary. But in such case we have no possibility
to proceed another response format.

Made response_key parameter optional. So if this parameter missed,
method will use all response, instead of response[response_key] if
client will have no possibility to provide required response dictionary.

related to blueprint common-client-library-2

Change-Id: I157c4bdea6243dd286ec6103494ede0d69ae4268