Update Heat core team

Remove imain and tomas-8c8, and add therve.

http://lists.openstack.org/pipermail/openstack-dev/2013-May/009660.html
http://lists.openstack.org/pipermail/openstack-dev/2013-May/009661.html

Change-Id: I4ded4636f26300437719f700a03439750bbddf2b