To support parallel cloud test execution, create and provide a random string to
cloud integration tests. The variable 'resource_prefix' can be used in cloud
roles and during resource cleanup to safely create/destroy cloud-based
resources.
Additional changes include:
* The roles test_ec2_key and test_ec2_group were updated to use to
{{resource_prefix}}.
* Additionally, the Makefile was updated to set resource_prefix to a random
string. The Makefile will also use 'resource_prefix' during cloud_cleanup.
* All test_ec2* roles were updated to add 'setup_ec2' as a role dependency.
