devstack plugin fails to remove ipa build, and doesn't build a new IPA when you restack

I recently encountered an issue where my IPA ramdisk was not rebuilt when I told DIB to build a ramdisk.

Some digging through the plugin revealed that the plugin only triggers the build if the file does *not* exist.

https://github.com/openstack/ironic/blob/master/devstack/lib/ironic#L3175

So clearly, the answer here is either on unstack, or most likely clean.sh trigger, to somehow nuke the cached IPA ramdisks so a new stack has fresh ramdisks.