We've seen build failures where replSetInitiate failed: The node cannot resolve its own hostname in the list of members, and therefore fails the command thinking that the replica set config does not include itself. A typical log line is:

[js_test:mixed_mode_sharded_transition_nossl] 2018-12-29T21:00:01.068+0000 c20022| 2018-12-29T21:00:01.068+0000 W NETWORK  [conn1] getaddrinfo("ip-10-122-5-158") failed: Temporary failure in name resolution 

It might be worthwhile to retry "Temporary failure in name resolution" in the server code itself. Meanwhile, add a retry loop around replSetInitiate in replsettest.js, the same as the retry we added around replSetReconfig in SERVER-27551.