Updated bb values.yaml to have a global value for image pull policy for all core packages

Package Merge Request

Package Changes

• Updated chart/values.yaml to include a global value for imagePullPolicy for all core packages (imagePullPolicy:{})
• Updated the values.yaml for all the core package template/spec in BB to make sure the global value is passed down to all core packages
• Updated image_pull_policy.md doc to show how to use the new global value for the core packages

For Issue

Closes #897 (closed)