Scan slurm job improvements (BUGZ-3966)

• Parts of the in-code documentation is improved, and follows more the condor backend example.
• The function handle_exitcode had an almost duplicate function handle_exitcode_cancelled. As the state is already established when calling handle_exitcode and stored as part of the message in the reason variable, there is no need for a separate function to handle that case, a simple if-test in handle_exitcode will do.
• The slurm query retry mechanism from !1211 (merged) is now configurable with a new option in the lrms block: slurm_query_retries. The default value is 0.