Make filesystem of root partition configurable

Make filesystem of root partition configurable

A new parameter, filesystem, is introduced which can be used to specify
the filesystem type of the root partition from instances. It defaults
to 'ext4'.

This also required to update the API version of the OS definition, as
instance parameters are only supported from version 20 on.

This fixes issue 464.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>