Add support for Gentoo Linux

Identify images of Gentoo Linux inside storage pools and local files,
and add a remote template for the latest version of Gentoo.

According to http://www.gentoo.org/main/en/name-logo.xml, some sentences
were added to Kimchi's terms of services regarding Gentoo trademark and
media ownership.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>