all: Move provisioning domain variable to fragments, Add placeholder validation, improve rsync dest

[marcelmamula]

Changes

1. Add validation for variables containing placeholder values to fail playbook before it proceeds with using them.
2. Move sap_vm_provision_dns_root_domain to platform fragment vars to ensure it does not trigger placeholder check when running for existing_hosts
3. Update RSYNC dest regex to ensure it is always pointing towards directory.
4. Move up sap_software_product into key variables to ensure improved visibility for users
5. Fix onehost > sandbox product directiory definition

[sean-freeman]

lgtm 👍