provisioning/vmware: simplify example, provide authentication tip

[Okeanos]

As discussed in coreos/fedora-coreos-tracker#1158 the examples for VMware vSphere now:

• tell users about alternative authentication methods
• give simpler instructions for a hello-world run
  • reuse of library items is relegated to an optional section

[dustymabe]

Looks like @lucab originally wrote this documentation. Anybody else familiar with VMWare enough to review this?

cc @mike-nguyen @bgilbert

[jlebon]

And thanks @fifofonix for testing and providing feedback!

[dustymabe]

Thanks @fifofonix @Okeanos !