Add NOMAD_SHORT_ALLOC_ID to alloc environment

[mikenomitch]

Proposal

Currently, Nomad exposes NOMAD_ALLOC_ID to the alloc's environment and this value can be interpolated in the docker hostname.

Given hostname length constraints, it would be nice if the NOMAD_SHORT_ALLOC_ID could be exposed as well and interpolated in this value.

Use-cases

Allow user to set up a docker hostname with a prefix that is of variable length (and not worry about it being too long).

[github-actions]

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