isodir as mirror in source list

Don't set the local isodir as mirror for the source list.

1. The sources.list.tmpl isn't applicable for a cdrom scheme:

2009-02-01 13:52:21,400 DEBUG Setting up sources.list
2009-02-01 13:52:21,545 DEBUG Output from template 'VMBuilder/plugins/ubuntu/templates/sources.list.tmpl': deb /media/vmbuilder_inst_image hardy main restricted universe
deb http://archive.ubuntu.com/ubuntu hardy-updates main restricted universe
deb http://security.ubuntu.com/ubuntu hardy-security main restricted universe

which will be end up in this:

Traceback (most recent call last):
  File "./vmbuilder", line 29, in <module>
    VMBuilder.run()
  File "/home/ar/vcs/lp/vmbuilder/VMBuilder/__init__.py", line 66, in run
    frontend.run()
  File "/home/ar/vcs/lp/vmbuilder/VMBuilder/plugins/cli/__init__.py", line 67, in run
    vm.create()
  File "/home/ar/vcs/lp/vmbuilder/VMBuilder/vm.py", line 468, in create
    raise e
VMBuilder.exception.VMBuilderException: Process (['chroot', '/tmp/vmbuilderNzG14l/root', 'apt-get', 'update']) returned 100. stdout: , stderr: E: Malformed line 1 in source list /etc/apt/sources.list (URI parse)

2. IMO it doesn't make sense to at it to the sources list because the apt-get update is done in the chroot and has no access to the mounted iso and the final created vm may be running also in a environment where the iso isn't reachable.

I removed it in VMBuilder::plugins::ubuntu::dapper::Dapper::install_mirrors (see attached patch)