[test_ec2*] cloud integration test updates

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.