feat(rest secu config): split REST API security configuration files to support: (#163)

* feat(rest secu config): split REST API security configuration files to support:

* when reading from a .properties files, also read from -custom and -internal
files if they exist
* when writing a property, do it in the -internal file

Closes [BS-16126](https://bonitasoft.atlassian.net/browse/BS-16126)

* feat(rest secu config): code review feedbacks

* Take code review feedbacks into account