labs_vmbuilder: copy all apt configuration
The vmbuilder postinst copy has a manually maintained list of apt
configuration files to copy from /etc/apt/*.d/. Whenever new files are
populated there, we might miss them.
Th copy step is defined in VMBuilder plugins/postinst/__init__.py and is
util.run_cmd('cp', '-LpR' ...
Replace manually list of files with a list of directories.