feature: add new option for generate default config file in default path

[ShahriyarR]

As discussed in #446 it can be valuable to add an commandline option to generate the default config file is this was somehow eliminated by setup.

--generate-config-file flag option was added. Sample run will be something like this:
autoxtrabackup --generate-config-file --verbose
2021-10-03 19:03:07 INFO [autoxtrabackup:278] Default config file is generated in /home/shako/.autoxtrabackup/autoxtrabackup.cnf
2021-10-03 19:03:07 INFO [autoxtrabackup:313] Xtrabackup command history:
2021-10-03 19:03:07 INFO [autoxtrabackup:315] ['command', 'xtrabackup_function', 'start time', 'end time', 'duration', 'exit code']
2021-10-03 19:03:07 INFO [autoxtrabackup:316] Autoxtrabackup completed successfully!

Closes #446

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.94%.

Quality metrics	Before	After	Change
Complexity	11.81 🙂	12.12 🙂	0.31 👎
Method Length	78.42 🙂	80.32 🙂	1.90 👎
Working memory	19.75 ⛔	21.27 ⛔	1.52 👎
Quality	56.47% 🙂	55.53% 🙂	-0.94% 👎

Other metrics	Before	After	Change
Lines	427	437	10

Changed files	Quality Before	Quality After	Quality Change
mysql_autoxtrabackup/autoxtrabackup.py	46.83% 😞	45.55% 😞	-1.28% 👎
mysql_autoxtrabackup/general_conf/generalops.py	79.13% ⭐	81.02% ⭐	1.89% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
mysql_autoxtrabackup/autoxtrabackup.py	all_procedure	27 😞	619 ⛔	46 ⛔	11.23% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
mysql_autoxtrabackup/general_conf/generalops.py	GeneralClass.backup_archive_options	4 ⭐	126 😞	7 🙂	68.28% 🙂	Try splitting into smaller methods
mysql_autoxtrabackup/general_conf/generalops.py	GeneralClass.backup_options	0 ⭐	126 😞	5 ⭐	77.29% ⭐	Try splitting into smaller methods

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