Update functional test _get_stack
Update functional test _get_stack

After change in heat ``StackManager.list()`` returns stack lists in
which stacks doesn't has a description method. So murano needs to use
``StackManager.get(stack_id)`` instead.

Closes-Bug: #1591180

Change-Id: Ia7c17e7415176042a5d6a7f8af2318baa8675278

openstack/murano,DavidPurcell/murano_temp,openstack/murano,DavidPurcell/murano_temp,DavidPurcell/murano_temp,DavidPurcell/murano_temp