Fix horizon config under Apache 2.4

Apache 2.4 now uses mod_authz_host for acces control. Horizon's
Apache config needs an update to allow access to its directory,
otherwise a 403 will be returned.
This change replaces a similar previous fixes done for Fedora 18
and Ubuntu 13.10, since this is an Apache version issue, not a
distro-specific one.

Change-Id: Iecc17600d8e1aae6a7b0929b1493d712c307616f
Closes-Bug: #1243075
diff --git a/functions b/functions
index 0a73b9f..5ca5eee 100644
--- a/functions
+++ b/functions
@@ -557,6 +557,18 @@
     [ "($uname -m)" = "$ARCH_TYPE" ]
 }
 
+# Checks if installed Apache is <= given version
+# $1 = x.y.z (version string of Apache)
+function check_apache_version {
+    local cmd="apachectl"
+    if ! [[ -x $(which apachectl 2>/dev/null) ]]; then
+        cmd="/usr/sbin/apachectl"
+    fi
+
+    local version=$($cmd -v | grep version | grep -Po 'Apache/\K[^ ]*')
+    expr "$version" '>=' $1 > /dev/null
+}
+
 # git clone only if directory doesn't exist already.  Since ``DEST`` might not
 # be owned by the installation user, we create the directory and change the
 # ownership to the proper user.