Configure a root disk & ephemeral disk for baremetal nodes

The current NODE_DISK variable (or disk option in testenv.json file)
specifies the size of the disk at our disposal. Add new ROOT_DISK
variable that specifies the size of the root partition on the the disk
and we then calculate and assign the remaining disk space to the
epheramal disk that will store all persistent data for baremetal nodes.

Change-Id: I1a941e1120fd4091311b7f80e145c47715ec2465
Merge "Configure a root disk & ephemeral disk for baremetal nodes"