Cleanup config data when draining nodes

Currently, when draining nodes we reset their master candidate flag, but
we don't instruct them to demote themselves. This leads to “ERROR: file
'/var/lib/ganeti/config.data' should not exist on non master candidates
(and the file is outdated)”.

This patch simply adds a call to node_demote_from_mc in this case.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>