Add option to extract ISO file URL from a remote file

The current Gentoo template automatically pulls the latest ISO for Gentoo by requesting http://distfiles.gentoo.org/releases/amd64/autobuilds/latest-install-amd64-minimal.txt (or http://distfiles.gentoo.org/releases/x86/autobuilds/latest-install-x86-minimal.txt) and extracting the path and name of the ISO file the text file.

@mpapis has suggested (ref. https://github.com/jedi4ever/veewee/pull/1018) introducing the following mechanism to Veewee for extracting the information:

```ruby
:iso_src => [:gentoo, 'http://distfiles.gentoo.org/releases/amd64/autobuilds', 'latest-install-amd64-minimal.txt'],
```