Previously we assumed the existence of some default objects that most
Opsworks users have because the Opsworks console creates them by default
when a new stack is created.
However, that meant that these tests wouldn't work correctly for anyone
who either had never used Opsworks via the UI or who had never accepted
the default of having the console create some predefined IAM objects to
use. It may also have led to some weird failures if a particular user had
customized the settings for these default objects.
Now the tests create suitable IAM roles, a policy and an instance profile
and use these when creating Opsworks stacks, avoiding any dependency
on any pre-existing objects.
This fixes#3998.
"Stack" is the root concept in OpsWorks, and acts as a container for a number
of different "layers" that each provide some service for an application.
A stack isn't very interesting on its own, but it needs to be created before
any layers can be created.
