[Installation] Improve setfacl options

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | No Symfony involved
| Fixed tickets | -

Improve setfacl options to avoid existing files gaining unwanted x-bit permission.

When setting the ACL for all existing files and directory recursivly, an uppercase "X" should be used in the permission options. It means granting the x-bit only to those files which already have it. Otherwise existing files like .gitkeep or existing cache and log files are provided with the x-bit.

See setfacl man page for more details (section "ACL ENTRIES"): 
http://manpages.ubuntu.com/manpages/precise/en/man1/setfacl.1.html