Improvement/make pull policies 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 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?

- [x] Documentation created/updated
- Tests
  - [x] 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

See merge request !393