rwanda: URLs fail sometimes due to DNS lookup error

[yolile]

From Slack:

There are some issues with the umucyo.gov.rw DNS.
I am seeing a number of "Horizontal Referrals" where DNS look-ups are pointing back to the same servers (and not any closer to a response).
I also experienced a timeout checking one of the nameservers directly, but after retrying it came back online.

We could try setting a get_retry_wait_time for the Rwanda spider to X seconds and see if that helps, but ideally, Rwanda should solve this issue with their DNS configuration cc @allakulov