Session.construct will create a session based upon the kwargs that used
to be passed to a client __init__ function. This will allow clients an
easy path to providing compatibility with deprecated arguments.
Make use of the function throughout discovery. Discovery was initially
released prior to the session object being completed and was therefore
handled with the same arguments as a client. Instead we should use a
session object so use the conversion function to convert those kwargs
into a session object if one is not provided.
Change-Id: I8dc1e0810ea6ebc6ea648ec37d7881825c566676
1263bd7c3a