e2e: make reboot test safe against new nodes joining mid-test
e2e: make reboot test safe against new nodes joining mid-test

hongchaodeng/bootkube,ericchiang/bootkube,hongchaodeng/bootkube,hongchaodeng/bootkube,ericchiang/bootkube,dghubble/bootkube,dghubble/bootkube,diegs/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,diegs/bootkube,dghubble/bootkube,diegs/bootkube,ericchiang/bootkube