Cancel wf engine via SshExecutor

It should be possible to terminate a running workflow run that is executed via the SshExecutor on a remote host.

• The workflow engine process needs to be terminated gracefully. It should get the opportunity to terminate running workload jobs that the engine submitted itself.
• Please track, whether the running Slurm/LSF cluster jobs submitted by the workflow engine (Snakemake, Nextflow) are also terminated. If not, make issues with some logs of your tests.
• The state in WESkit needs to be set to CANCELLING and later CANCELED.

Probably a SIGINT should be sent to the process. Possibly, after some (configurable) grace-period, if the engine process was not yet terminated a SIGTERM or SIGKILL should be sent.