NOMAD_PORT_<label> regression

[jazzyfresh]

for #6159

Behavior

• In 0.8.*, we set the port mapping in the task's environment variables.
  
• In 0.9.0, we do not override the port environment variables with the port map, so NOMAD_HOST_PORT_<label> and NOMAD_PORT_<label> are the same.
  
• In this fix, NOMAD_PORT_<label> gets set to the correct value from the port mapping, so the behavior matches 0.8.*

Todo

• docker driver
• rkt driver
• qemu driver
• tests

The rkt driver in 0.8 wasn't setting port maps correctly, as it didn't implement Prestart driver config part [1]; see qemu[2] implementation for comparison

[1] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/rkt.go#L396-L398
[2] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/qemu.go#L186-L205

[schmichael]

🎉

[github-actions]

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