Properly update iv_name of disks while changing templates

Trasforming the disk of an instance from DRBD to plain did not properly
update the iv_name of disks, leaving it to None.

This commit modifies _ConvertDrbdToPlain to properly set the iv_name
variables.

An analogous fix is not required in the opposite function,
_ConvertPlainToDrbd, because it uses _GenerateDiskTemplate, that already
takes care of doing that.

Fixes Issue 403.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

lib/cmdlib.py

@@ -14043,6 +14043,7 @@ class LUInstanceSetParams(LogicalUnit):
     # update instance structure
     instance.disks = new_disks
     instance.disk_template = constants.DT_PLAIN
+    _UpdateIvNames(0, instance.disks)
     self.cfg.Update(instance, feedback_fn)
     # Release locks in case removing disks takes a while