openvdc log fails on instances with FAILED state

[Metallion]

Problem

When we have an instance with state FAILED.

> ./openvdc show i-0000000005
{
  "ID": "i-0000000005",
  "instance": {
    ...
    "last_state": {
      "state": "FAILED",
      "created_at": "2017-07-05T09:08:41.181030773Z"
    },
    ...
}

We are no longer able to access its log. This makes debugging very difficult.

> ./openvdc log i-0000000005
FATA[0000] Error streaming log                           error="rpc error: code = 2 desc = cl.GetLog: application could not be found"

Cause

When an instance reaches FAILED state its mesos job is cleared. Mesos's log API no longer allows us to access it.

Solution

If there is no way to use the log API, rewrite the openvdc log <instance-id> command to be able to fetch the mesos logs directly from the executor machines.