Add ephemeral_mb record to bm_nodes

As the first step in making  baremetal support ephemeral partitions the
baremetal db needs to keep track of the ephemeral disk size.

Change-Id: I990f02a37799f4d9a3bde39a6058641a4a5314a4
Partial-Bug: #1238311
Merge "Add ephemeral_mb record to bm_nodes"