provide a way to override at admin level some plugins configuration available in plugin config.ini

[thefab]

like the number of workers for example

[thefab]

An idea (not ready):

• in a plugin, you have a metadata.ini with static/metadata things (version, license, cmd_and_args...) => all these parameters are set at dev level (no way to override them at deployment level) and you can't add some extra keys here (fixed scheme)
• in a plugin, you have a {plugin_name}.ini file with all parameters which can be overriden by an administrator at deployment level => this file includes some standard parameters (numprocesses, rlimit*...) but also dev defined specific parameters
• all parameters in {plugin_name}.ini are automatically changed into env var MFSERV_PLUGIN_{PLUGIN_NAME}_{GROUP}_{KEY}
• plugin code use only env vars and never read metadata.ini and {plugin_name}.ini
• [app_xxxx] and [step_xxxx] in {plugin_name}.ini are special as they are linked to corresponding groups in metadata.ini
• maybe we should introduce a [app_common] or [step_common] in {plugin_name}.ini to avoid this (of course one can add [app_xxx] to override a setting for just one app)?