Add Option for Member Health Maintenance to OS::Heat::AutoScalingGroup

The motivation for introducing this change is the lack of positive
affirmation in https://review.openstack.org/#/c/121824/ that there
will be a reasonable transition period during which both
``OS::Heat::HARestarter`` and some newer better alternative are
available. You can also see in the discussion in
http://eavesdrop.openstack.org/meetings/heat/2014/heat.2014-10-01-12.01.log.html
a desire to keep open the possibility that HARestarter will be removed
without a 1 release transition period. The proposed change is to
introduce, at least 1 release before HARestarter is removed, a wrapper
that (a) obscures the use of HARestarter, (b) has an interface that is
narrow enough that we are confident that it will be reasonable to
implement in the not clearly seen future, (c) has enough functionality
that current users of HARestarer can use the wrapper instead. In
addition to those three sharp criteria, there is also a desire to make
the wrapper be something that we would be comfortable with users using
indefinitely, not just to smooth over a transition away from
HARestarter. This enables current users to switch to using the
wrapper over a transtion period of at least 1 release before
HARestarter is removed. Once users have made this transition, we can
remove HARestarter and change the implementation behind the wrapper
simultaneously without disturbing users.