Fix configsvr with user/pass env vars on 5.0 & 6.0

[yosifkit]

Since 5.0, setting the sharding.clusterRole/--configsvr requires it to be part of a replica set so this fixes part of #509 (for --configsvr). Not certain we need to do anything for --shardsvr since they shouldn't use the entrypoint user/pass or initdb.d scripts there.

BadValue: Cannot start a configsvr as a standalone server. Please use the option --replSet to start the node as a replica set.
try 'mongod --help' for more information

Technically, adding --auth (via adding user/pass) also requires a keyFile for a replset, but that is something the user needs to add (not something we can fix in the entrypoint)

$ docker run -it --rm \
-v "$PWD/data:/data/configdb" \
-v "$PWD/keyfile:/keyfile:ro" \
--user [data & keyfile directory owner] \
--env MONGO_INITDB_ROOT_USERNAME=admin \
--env MONGO_INITDB_ROOT_PASSWORD=password \
--name mongo-cfg \
mongo:6.0 --configsvr --replSet rep --keyFile /keyfile/test.key

(I'd recommend users also set --hostname when using replica sets depending on how they set up members in their replica configuration rs.init(...).)