Do not use DatasourceNone for precise cloud-init

The 'None' Datasource does not exist in precise 12.04 (which uses
cloud-init 0.6.3), so conditionally ensure that we are not including
this in the datasource_list in case we're building a precise image.

Partial-bug: 1329068
Change-Id: I6efb91e17bf896bbf7d67c31c4eb5098d5d61a71
Merge "Do not use DatasourceNone for precise cloud-init"