Add variable $standard_version to apache class.

This will hold the version (major.minor, e.g. "2.2" or "2.4") of
Apache httpd that comes default with each operating system, so that
other parts of this module can do things differently depending on
the httpd version.  Currently we know which httpd comes with EL-6
and EL-7.

Note that this might not be the version that actually gets installed,
if the user has extra package repositories configured.  We could
possibly let users declare which version is used with a parametrized
class, but this will have to do for now.

manifests/init.pp

@@ -15,6 +15,23 @@ class apache
     $configfile = '/etc/httpd/conf/httpd.conf'
     $configdir  = '/etc/httpd/conf.d'
     $logdir     = '/var/log/httpd'
+
+    # The version of httpd that comes standard with an OS.
+    # This might not be the version that will actually be installed, though,
+    # but it is the best we can do.
+    # FIXME: Maybe allow users to override this with a parametrized class?
+    case "${::operatingsystem}:${::operatingsystemrelease}"
+    {
+	/^(RedHat|CentOS|Scientific):6(\..*)?$/: {
+	    $standard_version = '2.2'
+	}
+	/^(RedHat|CentOS|Scientific):7(\..*)?$/: {
+	    $standard_version = '2.4'
+	}
+	default: {
+	    $standard_version = ''
+	}
+    }
 }