no metrics from windows nodes available in Grafana when win_prefix_path is set on a windows cluster

[sirredbeard]

Backport of #79

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

Steps to reproduce (least amount of steps as possible):
on 2.5.8-rc18

• chart Repo commit

status:
  branch: dev-v2.5
  commit: 766d0ea8d73bb6a727a72c4a89fa4ce730479239

• Deploy a windows cluster with win_prefix_path set to 'c:\host\opt' (1 etcd+control, 3 linux worker nodes and 3 windows worker nodes)
• when the clusters comes up Active, deploy monitoring v2 chart on the cluster
• All the workloads come up Active, but no metrics from windows nodes are seen in Grafana and the Prometheus targets show windows nodes as Down

Expected Result:
Metrics from windows nodes should be available in Grafana.

Other details that may be helpful:

Environment information

• Rancher version (rancher/rancher/rancher/server image tag or shown bottom left in the UI): 2.5.8-rc18
• Installation option (single install/HA): HA

[sirredbeard]

Related to rancher/wins#41