[install] Heat updates for Mitaka
[install] Heat updates for Mitaka

Update launch an instance for heat.

1) Simplify by assuming use of CirrOS image.
2) Add ``mykey`` key so instance can use public key
   authentication.

Change-Id: If2b841ea86253923512af0eb0d5cc69c2c6d64d8
Implements: bp installguide-mitaka

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals