Only include yum packages on YUM platforms

It is unnecessary to include the yum cookbook on Ubuntu systems and often causes hard to trace constraint issues on Ubuntu/Debian-servers such as:


================================================================================
Error Resolving Cookbooks for Run List:
================================================================================
 
 
Missing Cookbooks:
------------------
Could not satisfy version constraints for: yum
 
 
Expanded Run List:
------------------
* opsworks_initial_setup
* ssh_host_keys
* ssh_users
* mysql::client
* dependencies
* ebs
* opsworks_ganglia::client
* opsworks_stack_state_sync
* rabbitmq_cluster
* deploy::default
* test_suite
* opsworks_cleanup
 
 
[2014-06-16T18:17:43+00:00] ERROR: Running exception handlers
[2014-06-16T18:17:43+00:00] ERROR: Exception handlers complete
[2014-06-16T18:17:43+00:00] FATAL: Stacktrace dumped to /var/lib/aws/opsworks/cache/chef-stacktrace.out
[2014-06-16T18:17:43+00:00] ERROR: 412 "Precondition Failed"
[2014-06-16T18:17:43+00:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)