Create a heat.enabled feature flag

As we are not quite ready to ship heat wholesale, add a heat.enabled
feature flag. This moves heat/tasks/main to heat/tasks/heat_main and
conditionalizes the include. Also update envs to support/not support
heat.

Also sneaking in a defaults.yml change for memcached.
Merge pull request #336 from craigtracey/heat_feature_flag

Create a heat.enabled feature flag