cml-runner: Add an option to reuse only idle runners

Today, the --reuse option of cml-runner does not care if the runner is in use or not. While it is convenient when multiple jobs can be run on the same runner, some job needs a dedicated runner to work (jobs that take the runner resources).

However, we could benefit from runner reusability for this kind of job by introducing a --reuse-idle option to the runner. This option would reuse a runner only if it is idle (option naming to be challenged).

[casperdcl]

use case explanation:

1. cml runner --labels=xyz --single: con: often have to wait 5min+ to provision
2. cml runner --labels=xyz --reuse: con: no parallel jobs (queue waiting for one runner)
3. cml runner --labels=xyz --reuse-idle: this feature request: no need to wait to provision iff another job has just completed (so an idle runner is available)

[0x2b3bfa0]

So true, we already face this possible race condition. I guess that it would be a minor issue to live with: the worst case would imply feeding a single runner with two tasks serially.