OAR: add async to _submit_job method

[msimonin]

_submit_job is awaited by the Job.start.

[lesteve]

Thanks a lot for fixing this! This bug was very likely added with the SpecCluster refactor (#307) and not noticed since the 0.7 release ...

I added an entry in the changelog, so merging this!

[lesteve]

As discussed IRL, if you ever get the chance to add CI for OAR, it would be super appreciated!

I would also make OAR more of a first citizen in dask-jobqueue (as SLURM, SGE and PBS) and help avoid similar problems in the future. We use docker-compose for your CI, you can have a look how it is done for SLURM, SGE and PBS: https://github.com/dask/dask-jobqueue/tree/master/ci.

While I am at it, one of the limitation of OARCluster: in the OAR cluster I have access to, oarsub is not installed on the compute nodes. My understanding is that this is the default setup in OAR.

If you create your OARCluster on a compute node (rather than on the login node, which is easier but not completely best practice compliant), a work-around as the one mentioned #333 (comment) is probably necessary.

If it turns out that oarsub is not installed on the compute nodes on your OAR cluster either, we could add a parameter to OARCluster e.g. login_node_host or something similar to make OARCluster easier to use.