Provide a default empty variable for 'bootstrap_host_data_disk_device'.
Tasks which make use of this variable within with_items lists have been
updated to evaluate that it has been set before executing.
'with_' clauses are evaluated before 'when' clauses. In Ansible 1.9
tasks are silently skipped when a variable within a 'with_' clause is
undefined, Ansible 2 provides a deprecation warning.
Change-Id: I66d7c15a43a70237b35d8ba7325d24e3b7206208
58e9c8db6e