Reduce rspec retries

What does this MR do?

Lowers the number of times QA and unit tests are run to 2 (1 initial and 1 retry).

Slack conversation that led to this MR: https://gitlab.slack.com/archives/C3JJET4Q6/p1554284893067000

Failing pipelines takes a log time since we were retrying a unit test job upto 2 (CI job retry) * 4 (rspec-retry) = 8 times. This is causes a lot of running pipelines.

Since we now have quarantining process in place, we can rely less on retries. But we cannot, at this stage, remove rspec-retry gem completely since the flaky tests detection mechanism depends on it. See this comment from Rémy

Conversation on previous MR for context

Moved from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26909

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• [-] End to end tests added if this MR has ~"Requires e2e tests" label

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

QA backend Quality