Make pull policies usage clear

What does this MR do?

Describes the usage details and cases of pull_policy parameters: always, if-not-present and never. It also change code a little to remove unused policy check and log specific exceptions to make clear what and why happened while determining the pulling policy.

Why was this MR needed?

After adding support for private docker images in !386 (merged) and fixing inconsistence of behavior of always pull policy we introduced a change that breaks workflows based on previous bad implementation of always pull policy handling. This MR adds documentation and cosmetic changes in code to make clear which pull policy should be used in what cases.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #1905 (closed)