(Re)move preamble processing. Now in Vcloud::Launcher::Preamble.

configure_guest_customization_section now just takes a single pre-templated/pre-processed script that will be passed to Fog.

This is the counterpart to https://github.com/gds-operations/vcloud-launcher/pull/51

Logic has been moved to Vcloud::Launcher::Preamble with a small piece around the returning of empty preambles in error states in Vcloud::Launcher::VmOrchestrator.