Add retry on exception to mattermost login spec

What does this MR do?

Add retry for mattermost login spec according to discussion here https://gitlab.com/gitlab-org/quality/nightly/issues/112

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
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• Separation of EE specific content