Use pkg_resources to detect version of cloud-init

Previously the present of an API call only present in cloud-init 0.6.x
was used to determine whether or not cfn-userdata would have been executed
or not. The API call was removed in 0.7.x. This Fixes bug #1103793

Change-Id: I27129f8ae6144ae7172b7159ed70fc5f0d7d07b9