Allow distros to modify how interfaces are restarted

Some distros may not utilize "ifup/ifdown" scripts to restart
their network interfaces. This patch just adds another method to
the abstract distro class to allow that default behavior to be
overridden.
Add Gentoo Linux concrete distro class

Adds support for the Gentoo Linux distribution (http://gentoo.org)
to the Windows Azure Linux Agent. There currently does not exist a
Gentoo Portage package for WALA, so the install mechanism is the
same as the manual steps in the README.