Update docs for EL7/U16 to install Mongo 4.0

[punkrokk]

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 v4.0 instead, following the #968

See StackStorm/st2-packages#643 for the changes needed.