Add /proc/self/mountinfo to hostInfo response

Ticket SERVER-40507 requested adding Kubernetes Persistent Volume Claims to the FTDC stats. After a long investigation, they were found to already be there.

The problem was that there was nothing in FTDC binding the ephemeral block device name for the underlying Elastic Block Store device to the filesystem seen by mongod. So there was no way for an operator to interpret the FTDC stats and see this.

So we should add the mount table to the hostInfo. This way we can gather some clue as to why a disk would be seeing the I/O stats it's reporting.