Heat version to latest stable-juno
Heat version to latest stable-juno

We need latest stable-juno version of heat, which has for example
fixed stack-update.
Also adding a note about variable needed to be set, so this
reporefs are actually used.

bcrochet/instack-undercloud,bcrochet/instack-undercloud,dmsimard/instack-undercloud,agroup/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud,eprasad/instack-undercloud,agroup/instack-undercloud,dmsimard/instack-undercloud,eprasad/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud