make congress compatible under keystone v3 context

The current version of congress datasource drivers authenticate with openstack component client code under keystone v2. Therefore while communicating with keystone v3, congress will run into errors. I tested and encountered such errors on datasource drivers of nova, glance, keystone and neutron. I've already made the change accordingly to these datasource drivers to make them compatible with keystone v3. In short, instead of using authentication approach against client code with username and password, we need to use keystone session for authentication. Most of the components can be authenticated with keystone session directly but several other components cannot (for example glance will need to use token which can be extracted from keystone session). I've already fixed these aforementioned datasource drivers. However, congress developers may want to take keystone v3 into consideration before they release the code in the future.