Use better variable values for ssh_public_key

[fghaas]

• fix: Set ssh_public_key to null (not "") in heat/environment.yaml
  Without this change, if the user neglects to set the ssh_public_key
  stack parameter, they get an error message that is rather unhelpful to
  a new user:

  ERROR: The public_key property is required by the nova API version currently used.

  By setting it to null (instead of the empty string) in
  heat/environment.yaml, they instead get:

  ERROR: The Parameter (ssh_public_key) was not provided.

  ... which is a much more self-explanatory message.

• fix: Comment ssh_public_key in ansible/vars.yaml
  When setting ssh_public_key to null or the empty string, the Ansible
  playbook proceeds and completes successfully, but creates an invalid
  SSH keypair and thus an invalid authorized_keys file in the Pad Ramp
  host.

  Thus, comment out the variable in ansible/vars.yaml (leaving it
  undefined), which causes the playbook to fail outright because of a
  reference to an undefined variable.

• fix: Comment ssh_public_key in tf/vars.tfvars
  When setting ssh_public_key to null or the empty string, "tofu apply"
  proceeds and completes successfully, but creates an invalid SSH
  keypair and thus an invalid authorized_keys file in the Pad Ramp host.

  Thus, comment out the variable in tf/vars.tfvars (leaving it
  undefined), which causes OpenTofu to prompt for the missing variable
  (unless a TF_VAR_ssh_public_key environment variable is set).