Add 'rhsm_become' role variable

[robled]

Since this role requires root to run, it should provide a 'rhsm_become' variable that can be used in the event that the Ansible SSH user needs to elevate privileges.

Example: https://github.com/oasis-roles/system_repositories/blob/master/defaults/main.yml#L16

[tehsmyers]

Nope. Since this role requires root to run, it just becomes root every time (which the root user can itself do). Or...it should, but I apparently messed up in 29d4ff5 and need to fix it. But escalation in this role is not optional, so I won't expose that as a var.