Add graceful termination for ssh executor
Overview
For the ssh executor when we want to finish a process we simply send SIGKILL
which might lead to processes not being terminated properly as we've seen with other executors such as #6359 (closed) & #3376 (closed).
Proposal
Send a SIGTERM
first to the processes and then after a specific timeout send SIGKILL
. This will help processes and give them time to clean up their resources child/processes.