Put $bind_address in vhost config only if it's explicitly specified

Only add the 'ip' element to the $default_vhost_conf hash if it was explicitly
specified in the instantiation of the class.  This is because ip => undef gets
changed to ip => '' via the Puppet function API when ensure_resource is called.
For port => 80, that causes a conflict with the Apache module's default
port 80 vhost listener.

Change-Id: I504857aca99a7a3c42ee17242bcc0dea33f1b6e4
Closes-Bug: 1371345
Merge "Put $bind_address in vhost config only if it's explicitly specified"