rancher: Logging integration doesn't work if Docker Root is not default /var/lib/docker

What kind of request is this (question/bug/enhancement/feature request): Bug

Steps to reproduce (least amount of steps as possible):

1. Provision a server with Docker, but use a path other than /var/lib/docker for Docker’s root. For example /data/var/lib/docker
2. Create a new custom cluster and add host above.
3. Set the Docker Root Directory to /data/var/lib/docker
4. Enable the logging integration.

Result: The rancher-fluentd-logger is using /var/lib/docker as the dockerroot and will likely not be able to follow the symbolic links in /var/lib/rancher/rke/log/ to access the logs.

Other details that may be helpful:

Environment information

• Rancher version (rancher/rancher/rancher/server image tag or shown bottom left in the UI): v2.3.0-alpha5 (also reproducible on v2.2.4)
• Installation option (single install/HA): single install

Cluster information

• Cluster type (Hosted/Infrastructure Provider/Custom/Imported): Custom

• Machine type (cloud/VM/metal) and specifications (CPU/memory): t3.medium

• Kubernetes version (use kubectl version): 1.14.3

• Docker version (use docker version):

$ docker version
Client:
 Version:           18.06.3-ce
 API version:       1.38
 Go version:        go1.10.4
 Git commit:        d7080c1
 Built:             Wed Feb 20 02:24:22 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server:
 Engine:
  Version:          18.06.3-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       d7080c1
  Built:            Wed Feb 20 02:25:33 2019
  OS/Arch:          linux/amd64
  Experimental:     false

gz#6652