Fix races in sd.createMasterDir()

Change-Id: I6a79b620853663bc7ab7e33633bd6dc9f67759e6
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12806
Tested-by: oVirt Jenkins CI Server
Tested-by: Daniel Paikov <paikov@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>