core: don't throw an exception, when moving unresponsive VDS to maintenance

When VDS which is part of hosted engine cluster becoms non-responsive,
it throws an exception when running SetHaMaintenance command which
results in roll back of the whole maintenance command preventing
user from removing the broken VDS

Change-Id: Ibf9b489f73793ea1135a72bf1a9184e0822386d8
Bug-Url: https://bugzilla.redhat.com/1162998
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>