Restrict env update command to specific variable and values

[0xlianhu]

Restrict/simplifymyhoard_mysql_env_update script can only update the MYSQLD_OPTS variable with specific values.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 79.31%. Comparing base (ae06c2e) to head (0739d9d).

Files	Patch %	Lines
myhoard/update_mysql_environment.py	0.00%	15 Missing and 1 partial ⚠️
myhoard/myhoard.py	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   79.87%   79.31%   -0.56%     
==========================================
  Files          17       17              
  Lines        4496     4506      +10     
  Branches     1016     1018       +2     
==========================================
- Hits         3591     3574      -17     
- Misses        670      696      +26     
- Partials      235      236       +1     

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[rikonen]

Looks good to me except the value could in some cases contain extra trailing whitespace. I'd expect that to just get ignored but maybe it would be better to just construct the value the same way we used to as suggested in the other comment.