* Merge prechecks.yml and site.yml playbook
* Create empty precheck.yml into all roles.

Change-Id: I8a138558a26c0a2a66c5fd48ed37be657c99c1dd
Implements: blueprint condition-pre-check

- This change indroduces a merge_yaml action_plugin
- This change generates bifrost yaml configs.

Change-Id: I9814e6a5d55cbd46c4b60c06ed70ed54a575bd2f
Implements: blueprint bifrost-support

This partially implements iscsi and lvm2 support for cinder
in Kolla. Adds ansible code for iscsid and tgtd containers.

Change-Id: I2a4cfcf104397396ea61a09637d9ef3fc77b515f
Partially-Implements: blueprint iscsi-lvm2-docker

DocImpact

Partially-implements: bp kolla-reconfig
Change-Id: I9738e80960bcfbef18d1ef1b7942f81c45684e85

After introduction of pull action and turing every main.yml into
{{action}}.yml we lost ability to perform upgrade

Change-Id: Ie9fa2cd083b061033abc733fba53d54f9c55e393
Fixes-Bug: #1538210