Update deprecated dask-jobqueue names

Parameters from dask-jobqueue's classes (Job and JobQueueCluster) will change name soon. This follows those changes (job_extra to job_extra_directives and env_extra to job_script_prologue.

A config option (jobqueue.sge.job-extra) became invalid due to the name changes and returned None which was not handled correctly, making the submit commands fail silently and the scheduler waiting for the jobs. The option is now renamed.