Pin tornado version in requirements file
Pin tornado version in requirements file

tornado needs to be >= 4.2.1, but less that 5.0.

Tornado 5.0 is introducing backwards-incompatible changes. Therefore,
we need to pin the version of tornado in base.txt until we can fix
supporting Tornado 5.0 in Salt.

Refs #45790

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt