Provide a default empty variable for 'extra_container_config'.
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: I3acd16a739cbdbed84d8d74de2b78b52577d4ee2
773dc541dd