Previously, low level Nomad job creation was done in the environment manager.
It used many functions of the nomad package so we felt like this logic
better belongs to the nomad package.
As we can't control which allocations are destroyed when downscaling a job, we decided
to use Nomad jobs as our runners. Thus for each runner we prewarm for an environment,
a corresponding job is created in Nomad. We create a default job that serves as a template
for the runners. Using this, already existing execution environments can easily be restored,
once Poseidon is restarted.
