Use ${APACHE_LOG_DIR} instead of /var/log/apache2
APACHE_LOG_DIR note added

Added a note, that for systems, which support the APACHE_LOG_DIR env, the user can use it for the logging-path.
minor #4104 Use ${APACHE_LOG_DIR} instead of /var/log/apache2 (xamgreen)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4104).

Discussion
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Use ${APACHE_LOG_DIR} instead of /var/log/apache2

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none
It would be a better solution to use envs in the configfile.

Commits
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b8328c5 APACHE_LOG_DIR note added
8753f37 Use ${APACHE_LOG_DIR} instead of /var/log/apache2