pybal: use Apache <IfVersion> instead of os_version

The Apache configuration template was having conditionals based on an
$is_24 variable, that was set in the manifest based on the operating
system's version. That's not so pretty, so convert to Apache's
<IfVersion> syntax for switching between the different configuration
directives.

Change-Id: I43e55a017cb56b219e55c7fb4b825fa1ecfdb18b