Use "become: true" for users role

Use "become: true" for users role

This is needed if the users role is called manually.  Typically it's
called via the common role and "become: true" is inherited.

But in the case of just needing to update update the teuthology_user's pubkeys with @all.pub, just calling the users role manually is quicker.  sudo access is obviously required to modify users and groups.

Signed-off-by: David Galloway <dgallowa@redhat.com>