several improvements around idempotency

[jgoulah]

The main goal of this commit is to allow the script to be run multiple times without erroring.

- several fixes to make script more idempotent
- added AUTO_MOUNT_HOME to control whether we want our home dir mounted by default on /mnt
- send contents we are writing to fstab to moby console for better idea of what script is doing
- fix typo

[jgoulah]

forgot to mention in the commit msg, but removed the "Log into D4M as root" stuff because it didn't seem necessary (it appears its already root by default and the command "root" throws an error)

[rabidscorpio]

@if-drew Would love to get this merged when you get the chance.

[if-kenn]

I think you meant @if-kenn not Drew, he is one of our engineers, but doesn't make changes to this.

@jgoulah all the changes look good, my only concern is that Docker changed that the Moby VM now already has root logged in which before they didn't. While it provides not found, if they revert to needing a login then it will break things going forward. I will merge the request, but we could get bitten by them regressing on us.

[jgoulah]

@if-kenn understood, its a bit of a moving target, will just have to patch if things change. thanks for merging!

[rabidscorpio]

Thanks @if-kenn