In order to support virtual device role tagging that was introduced in
microversion 2.32, this patch adds:
* The 'tag' key to the --nic flag when booting an instance.
* The 'tag' key to the --block-device flag when booting an instance.
Change-Id: I1866c670994254bc2849b494e318f4ff8cc44eb7
Implements: blueprint virt-device-role-tagging