Add VM instructions, readability edits

[mounaiban]

This edit is related to open-horizon/open-horizon.github.io#56. It adds instructions to mgmt-hub/README.md for preparing virtual machines for use with the all-in-one environment.

The other changes are mostly about the following:

• Adding minimum system requirements
• Word count reduction on some of the existing text
• Changing all instances of all-in-1 to all-in-one

I have placed the VM instructions at the bottom of mgmt-hub/README.md to avoid getting in the way of the existing information. I hope there are no issues with this.

[joewxboy]

@bmpotter Any objections to merging this?

[mounaiban]

Sorry, I spotted a typo, so I had to make another commit to fix it. I took the opportunity to fix the formatting as well. I have just done a rebase, and the PR is good to go.