Add instruction for Heat OSTF tests.

Added instruction for Heat autoscaling and
new test scenarios for Heat component
to the post-install-healthchecks.rst.

Depends on https://review.openstack.org/#/c/62323/
that implements new Heat tests.

Change-Id: I17c68b26d90304b202c61387d82d8c56ccfdf550
Merge "Add instruction for Heat OSTF tests."