Delete created keypairs if nova boot fails

If the server fails to boot (perhaps due to quota issues) during
an image update, if nodepool created a key for that server it won't
be deleted because the existing keypair delete is done as part of
deleting the server.  Handle the special case of never having
actually booted a server and delete the keypair explicitly in that
case.

Change-Id: I0607b77ef2d52cbb8a81feb5e9c502b080a51dbe