test/packet: Update README.md
test/packet: Update README.md

This commit includes various updates:
  - Rework the formatting so that it is renderable and readable in the
    browser
  - Add missing instructions to create a ssh key pair
  - Clarify the terraform variables that need to be set
  - Use terraform > 0.13 syntax (drop the `.` from `terraform init .`)

Signed-off-by: Chris Tarazi <711c73f64afdce07b7e38039a96d2224209e9a6c@isovalent.com>

michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium