Fix a bug that was introduced when we shifted to using os_setup
directly (on the Cloudlab clusters); we were losing a lock out that allowed DeleteSliver() to run while in the middle of a CreateSliver(). This was resulting in a lot of email about node failures since the nodes were getting yanked out from underneath the CreateSliver(). From the user perspective, this did not matter much, since they wanted the slice gone, but it finally bothered me enough to look more closely.
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