provision.downburst: Tweak destroy return code
provision.downburst: Tweak destroy return code

Specifically, if the instance doesn't even exist, consider the destroy
op to have succeeded.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

ceph/teuthology,SUSE/teuthology,SUSE/teuthology,dmick/teuthology,SUSE/teuthology,dmick/teuthology,ktdreyer/teuthology,dmick/teuthology,ceph/teuthology,ktdreyer/teuthology