title: HAProxy - use_backend if it's available
body: <p>Is there a way to utilize <code>use_backend</code> with an ACL match, but, in the case the backend is unavailable (down,maint,etc), then use the default? </p>
  
  <p>For example:</p>
  
  <pre><code>    # Define hosts
      acl host_bacon hdr(host) -i ilovebacon.com
      acl host_milkshakes hdr(host) -i bobsmilkshakes.com
  
      ## figure out which one to use
      use_backend bacon_cluster if host_bacon
      use_backend milshake_cluster if host_milkshakes
      default_backend web-app-cluster
  </code></pre>
  
  <p>In the case above, if the bacon and milkshake backends have no available servers, to fall and use web-app-cluster?</p>
  
  <p>Thanks</p>