Configure horizon when using /etc/apache2.

The horizon image element doesn't appear to work when the http server
uses the /etc/apache2 directory.

Fix up one incorrect symlink, add one more, and remove default config
files which might obscure horizon on port 80.

Change-Id: I6427de66f73a5853a80ea75d753218172a078d9d
Closes-bug: 1281703
Merge "Configure horizon when using /etc/apache2."