Speed up obtaining Let's Encrypt certificates

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/28996

Currently Let's Encrypt workers are run only by cron job which is scheduled every 5 minutes, which makes obtaining time 10-15 minutes (worker need to be run 3 times).

This MR adds scheduling subsequent iterations right after finishing current one. Also make cron job run only every 10 minutes. That will result in 3-13 minutes overall time.

Another good idea would be to schedule first iteration right after user sets auto_ssl_enabled to true, but I'd leave this for future improvement.

Does this MR meet the acceptance criteria?

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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