Add kubelet DNS flags & api disable for DNS

This adds --cluster_dns and --cluster_domain flags to kubelet.  If
non-empty, kubelet will set docker --dns and --dns-search flags based on
these.  It uses the cluster DNS and appends the hosts's DNS servers.
Likewise for DNS search domains.

This also adds API support to bypass cluster DNS entirely, needed to
bootstrap DNS.
Use a helper bridge between k8s and SkyDNS

Rather than have to keep SkyDNS up to date with Kubernetes, use a
buddy-container "kube2sky" to watch kubernetes and sync Service records
into etcd for SkyDNS ot find.  This also adds namespace support.
Make DNS an option for cluster turnup