Make fullClone clone mode default. Use linkedClone only if explicitly specified.

Currently, linkedClone uses by default which cause silent diskGiB parameter ignorance
in case if Snapshot specified or clonee template has snapshots.
Disk size from vm template will be used in this case, which might be undesirable.
This commit changes default behaviour of controller, fullClone will be used by default,
linkedClone should be set explicitly.