Doc for GlusterFS Ganeti Support

This patch adds a design document for introducing GlusterFS Support in Ganeti.

Signed-off-by: Weiwei Jia <harryxiyou@gmail.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>
Fix node modify QA tests for large cluster

If the node count in a cluster exceeds the master candidate pool size
(which is 10 by default), it's no longer possible to remove the master
candidate role from a node without passing --auto-promote (even with
--force). This behavior is perfectly fine, but the TestModifyNode QA
test didn't account for this.

TestModifyNode is changed such that it first removes the master
candidate role from the tested node with --auto-promote. This will, in
big clusters, promote an additional node to a master candidate which
ensures that the following tests succeed.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>