Assign unique names with os-multiple-create.

This patch introduces a new configuration option,
multi_instance_display_name_template, which makes
it so that all instances are given a unique
display_name and hostname when requesting that
multiple instances get created at the same time.

Implements bp multi-boot-instance-naming

Change-Id: Id8e7dac1adadbb101dd1027807867ab13d3b7e80