Implements stack lifecycle plugpoints

Stack lifecycle plugpoints have been proposed in
http://summit.openstack.org/cfp/details/86
and
https://etherpad.openstack.org/p/juno-summit-heat-callbacks

Implements: blueprint stack-lifecycle-plugpoint
Change-Id: I8c7b5d0113392e54fe0f35933c2c10da277fd90b
Merge "Implements stack lifecycle plugpoints"