Updates to inventory generation and instance configuration

This change makes few updates to inventory generation mechanism,
where generated inventory.ini files are placed, and how they are
used during the deployment.
- move inventory generation out of create-stack role and place it
in newly created role, generate-inventory, for provisioner Heat.
This will help us when we work on generating generic inventory
by provisioners since both provisioners now use separate roles
to generate inventory.
- move tasks in configure-instances located in provisioner to
installer configure-targethosts playbook
- place generated inventory.ini under config_path to have it in
same place as before.
- symlink config_path/inventory.ini as engine/inventory/inventory.ini
instead of copying it so we have single source of inventory.

Change-Id: I53cd4b1e1c1f641afbb992cf08818bdfa4ab767e
2 files changed