Consolidate to /mnt/cook/sandbox as the sandbox directory for both mesos and k8s

[shamsimam]

Is your feature request related to a problem? Please describe.

It consolidates the sandbox directory for both mesos and k8s.

Describe the solution you'd like

We should either mount or create a soft-link to the common directory.
Mesos will need a soft link.
K8s pod spec will need the new directory.
In either case, the directory will need to be pointed to by the COOK_SANDBOX environment variable.