Added Reference Architecture document.

This patch adds the latest Reference Architecture in AsciiDoc format.

The sources have been converted from the existing DITA sources as of
Git commit a9372bf4ce from the master branch of the Midokura
Documentation repository: https://github.com/midokura/midonet-docs

Change-Id: I32e4f20b504908b840da16b4e1d38cbbeb181ac4
Signed-off-by: Jan Hilberath <jan@midokura.com>
Merge "Added Reference Architecture document."