Drop extra and env-extra requirements in core.py

[raybellwaves]

@mrocklin noted in #78 that all the options in jobqueue.yaml may not to required for local clusters. For example, I haven't used extra and env-extra in development of lsf.py.

At the moment these are required to be in core.py as https://github.com/dask/dask-jobqueue/blob/f7c565a2fe87e8829acba3c752a4da8d196c92ca/dask_jobqueue/core.py#L186L189 requires them.

We could drop these from core.py and require users to specify them in the local clusters.

[jhamman]

I'll come down against this proposal. I am confident that extra/env-extra will be useful to some LSF users. They are a key in adding flexibility to all of the jobqueue Clusters.

[guillaumeeb]

I'm in line with @jhamman here. I think its a generic functionnality required by some of jobqueue users, that need to stay in core.py.

[raybellwaves]

I can’t say I’m very opinionated on the matter. I’ll let @mrocklin chime in. Otherwise, welcome to close.