apache: ensure 'Include' wildcard globs always match

If apache2.conf tries to 'Include sites-enabled/*' but no files match the glob
pattern, Apache 2 issues the following error and refuses to start:

  apache2: Syntax error: No matches for the wildcard '*.conf' in
  '/etc/apache2/sites-enabled', failing (use IncludeOptional if required)

However, 'IncludeOptional' is not available on Apache 2.2.
Work around this by always provisioning a dummy file.

Change-Id: I445d652c189998a9f39efed807cb881dc2b0acee