Fix stage status shown for pipelines

What does this MR do?

Properly calculates a status of stage when shown on a list of pipelines

Why was this MR needed?

Previously we were calculating a status for all builds, including retried ones. This were affecting a overall stage status when a build were retried.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18896 https://gitlab.com/gitlab-org/gitlab-ce/issues/18150

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together