heat_keystoneclient: Move v3 path logic to constructor

Work out the v3 path in the constructor and set it as a variable
as it is needed in more than one place so it's better to just
do this once.

Change-Id: I6f8e887c0eeacd3b2e78a71e36fcd93850a144c2
Merge "heat_keystoneclient: Move v3 path logic to constructor"