Add blocking option to DeleteSlice(), which restores original behavior for people who want it. Set the creator_urn of image properly; we were not handling it in the presence of a speaksfor credential. Minor dataset tweaks.
Add blocking option to DeleteSlice(), which restores original behavior for
people who want it. Set the creator_urn of image properly; we were not
handling it in the presence of a speaksfor credential. Minor dataset tweaks.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome