fix: update a pod's IP if it already exists

[andrewrynhard]

In the case of a node going down and coming back up, a pod will retain its' name, but change its' IP address. Argus should update system.ips with the updated IP address.

time="2018-03-02T21:41:32Z" level=error msg="Failed to add pod \"kube-dns-6568cd868b-zjgxq\": [REST] [600] There is an existent host(id=64383, hostName=kube-dns-6568cd868b-zjgxq) with same display name(displayName=kube-dns-6568cd868b-zjgxq)"

• Argus Version:
  0.2.0-alpha.0
• Kubernetes Version:
  1.9.2
• Docker Version:
  1.12.6