Add prepend_script and append_script for job.resource

[Cloudac7]

It is sometimes in need of executing command lines before or after task submitted to startup or shutdown necessary environment. While most issue with prepend script required like this might be solved by source a static script on remote server (which might also be difficult when remote server is not so stable.), scripts to be executed after tasks finished might be difficult under current dpdispatcher resource.
Here in this PR, prepend_script and append_script parameters have been add to job.resource in form of List, in which a single line could be an item, like this:

"prepend_script": [
    "conda activate test_env",
    "export PATH=/path/to/package:$PATH",
    "send_an_email_to 114@514.com"
    "sleep 1919810"
]

and the expected output:

conda activate test_env
export PATH=/path/to/package:$PATH
send_an_email_to 114@514.com
sleep 1919810

Another little change is just add delay=True for logger to prevent the generation of dpdispatcher.log even if no log information output.

[Cloudac7]

For prepend_script, it could also be an optional choice for now existing module_list, source_list, envs, etc. But the latter ones could not cover all requirement for executing a script before tasks submitted. Also procedure of tasks could be customized by user instead of the predefined module -> source -> env one.

[codecov-commenter]

Codecov Report

Base: 58.63% // Head: 59.44% // Increases project coverage by +0.81% 🎉

Coverage data is based on head (390ff61) compared to base (eb86fc0).
Patch coverage: 71.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   58.63%   59.44%   +0.81%     
==========================================
  Files          26       27       +1     
  Lines        2879     2895      +16     
==========================================
+ Hits         1688     1721      +33     
+ Misses       1191     1174      -17     

Impacted Files	Coverage Δ
dpdispatcher/distributed_shell.py	17.64% <0.00%> (-0.88%)	⬇️
dpdispatcher/lsf.py	37.36% <ø> (+14.28%)	⬆️
dpdispatcher/machine.py	86.27% <100.00%> (+4.27%)	⬆️
dpdispatcher/submission.py	84.33% <100.00%> (+0.21%)	⬆️
dpdispatcher/_version.py	100.00% <0.00%> (ø)

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[Cloudac7]

Well, it might be my ignorance that not using string.join() for I just thought error was raised when empty list passed, while it won't. It also hint me that the unittests not include a tempelate rendering test, it might be a good idea to add one.

Well, local test for script generation was add for Slurm, but not including env and end. Here considering coverage, I just add one for LSF and thus test for other parts in script in a local way.

[Cloudac7]

This line is useless.

Fixed.