Edits to attach-data-disk-centos-vm-in-portal.md

- Update to provide notes on editing fstab and using UUIDs
- Other minor updates
Edits to CreateVirtualMachineLinuxTutorial.md

- Update changes from howto-attach-disk-windows-linux.md
Merge pull request #1752 from szarkos/fstab-doc-improvements

Linux fstab documentation improvements