Merge Gentoo templates into a single template

Merged the three 3 Gentoo templates into a single template with multiple YML configuration files.

The 4 possible configurations are:

* gentoo_amd64.yml
* gentoo_amd64_minimal.yml
* gentoo_x86.yml
* gentoo_x86_minimal.yml

The first is the default one.

The the YML configs include a non-standard value, `architecture`, which is used to automatically retrieve the ISO for latest version of Gentoo for that architecture.