No support for PersistentVolumeClaims in Kubernetes backend

[mvdbosch]

In a Google Kubernetes engine, it is not possible to manually set-up a shared workspace mount. The way to have a shared disk, is by using Persistent Volumes instead of the Host_path volumes (as is currently implemented in the api_client)

When I have time available, I will attempt to implement the following:

• configuration setting, which can set the option for host_path based or persistent volume based
• Persistent Volume implementation in the Kubernetes api_client

[dvenza]

For this issue too, feel free to issue a merge request once you are satisfied with your work.
Thanks!