Fix race condition with fast scans on setup

After setting up a new vendor, we print out information about the scan
which has been initiated. For some vendors however - notably the mock
vendor - it's possible that the scan might complete before we actually
try to print out it details.

This commit modifies do_describe() in vendors.py to use the newly-added
refresh_resource() method of the Navigator class. This method both
checks for the existence of the object and returns its properties in a
single operation. Thus, when do_describe() goes to access properties of
the scan object, it can do so safe in the knowledge that the information
is already available in local memory (previously it would throw an
exception on 404).