Support for the 'multi_instance_display_name_template' option was
removed in commit 0e43002c9 and booting multiple instances now will
simply result in a simple numerical suffix (-1, -2, -3, ...) being added
to the instance's 'display_name' and 'hostname' attributes.
We're now in a position to greatly simplify how we do this, so do just
that, removing some comments and updating some documentation that
referenced the removed option in the process.
Change-Id: If5369bcf4a6f7fc36190e153776e60446eb14040
2a969467dc