Support os variants

Support os variants

This is a Ganeti 2.1 design feature. It's not implemented yet in 2.1,
but its implementation in the debootstrap OS is totally backwards
compatible. If no OS_VARIANT environment variable is passed in,
everything proceeds as before, otherwise a instance-deboostrap tries
to load a config file from a specially configured directory, which can
be used to override any variable in the defaults file (which is sourced
at the beginning as it was before.

By default only the "default" variant is exported, which is shipped as
an empty config file.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>