Cope with '%' in password when waiting for migrations

[highfly22]

---

When there is a '%' in the sql connection url, wait-for-airflow-migrations container complains no permission to connect database.
This commit fix that issue by replacing '%' with '%%'.

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[boring-cyborg]

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[potiuk]

I thin the proposed solutin is not good - see the comment.

[potiuk]

I see!

[boring-cyborg]

Awesome work, congrats on your first merged pull request!