Change Stack timestamps to save correct info

Stack.updated_time is currently being treated as modification time of
the object in the database.  Since it's not a very useful thing to have
and the CFN API treats this value as "the last time an update call was
issued", it makes sense to have the same behavior here as well.

This changes the current real-time retrieval of created_time and
updated_time in favor of the attributes already present in the stack and
changes the information stored in these two fields to be the time the
stack was created and the time the stack was last updated by the user,
respectively.

Co-Authored-By: Anderson Mesquita <andersonvom@gmail.com>
Closes-Bug: #1193269
Change-Id: I1fd6586b827b3f2babb5af5c85f9de25da74fbb6
Merge "Change Stack timestamps to save correct info"