Optionally remove portage files, so that we can cache package and
keep the portage directory around, specifically for nodepool.
This also adds a section to the Gentoo readme about the variable
and renames the 00-gentoo-distro-name environment file to a more
appropriate name of 00-gentoo-envars.
Also brought up was the location of the gentoo-releng.gpg file,
this has been moved and the refrencing paths updated.
Change-Id: I20c91b36082828faa1ca481585acc5f9933211e1
