Only use Ec2 cloud-init data source for Ubuntu

Default to only having cloud-init query Ec2 on first boot for Ubuntu,
until cloud-init has been SRU'd to fix the CloudSigma data source issue
that causes Trusty boots to hang.

Change-Id: Icb3734d5ae78f4a0a6c0fae1af4a2ce3c809308c
Partial-bug: #1316475
Merge "Only use Ec2 cloud-init data source for Ubuntu"