Move MongoDB 3.4 -> 4.0 for EL7/U16 due to EOL

[arm4b]

While newer recently added platforms like Ubuntu18 and CentOS8 rely on MongoDB 4,
older systems like EL7 and U16 are still on MongoDB 3.4

MongoDB v3.4 has reached its EOL in January 2020.
Switch to MongoDB v3.6 MongoDB v4.0 instead, following the StackStorm/st2docs#968

st2docs, ansible-st2, Puppet and stackstorm-ha (StackStorm/stackstorm-k8s#97) deployment methods and st2 CI/CD will also need an update.