hack/quickstart: bump bootkube image to v0.3.8
hack/quickstart: bump bootkube image to v0.3.8

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