Add UserData feature

Adds a "UserData" field to the ProviderConfig. For supported provider
(most of the cloud providers) this allows serving userdata to the
instance.

Fix DO image creation

- Digital ocean only accepts images with certain extensions (such as
.qcow), add . in imagename; - Images may not be publicly shared, create
a pre-signed URL, it gets used immediately so no long lived URL is
needed.