Docs improvements

Fixes: bug #1204831

Change-Id: I503c320dbf91c8737adde95ac94ac392784d0de8
Validate image tags to contain required tags for plugin

Add method check_required_image_tags to validations/base.py which checks image tags to containing required tags
Add validation required tags for default image to cluster create validation and cluster create template validation
Add unit tests for this methods

Fixes: bug #1204410

Change-Id: Iff80057106fc73fc5f29b5a1f983f99b33840b73
Merge "Validate image tags to contain required tags for plugin"
Merge "Docs improvements"