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.
Merge branch '2.0' of https://github.com/Azure/WALinuxAgent into 2.0

Conflicts:
	waagent
Change the ATAPI module requirements in Gentoo

The code that handles ATAPI CD-ROM loading should probably be a bit
more fault-taulorant so that kernels with built-in ata_piix drivers
still can be provisioned, but for now just change Gentoo's distro
support to use modular ata_piix drivers like the other distros.
Merge pull request #35 from cvpcs/2.0

Changes to allow WALA to support Gentoo Linux