rhsm: add rhsm_release in environment for doc purpose
rhsm: add rhsm_release in environment for doc purpose

Document the rhsm_release in the rhsm environment so the variable is
easily discoverable when someone will build their rhsm env.

Depends-On: https://review.opendev.org/730339
Change-Id: I7899f65b1663d9b5cff1040d93186acff5ca8b0b

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates