Add apache::conf_dir, use it instead of params.

This is another change needed to make the httpd24 Software Collection
work on RHELish boxes.

Fix mod_systemd on RHEL/CentOS 6.

Puppet < 3.0 does not have unless.

Try another fix for Puppet 2.7 / Facter 1.7.

Add README and tests for ::apache::conf_dir.
Merge pull request #821 from smerrill/feature/fix-remaining-rhel6-scl-issues

Finish SCL support for RHEL/CentOS 6