Fix additional issue with _member_ role creation

I removed the '--tenant' option from the admin user/tenant
creation step because the latter needs only the admin role.
Also, I provided an explanation about automatic assignment
and/or creation of the _member_ role.

Change-Id: I036ae43b73c8ca469e04e8090e197d57a7a5f5d0
Closes-Bug: #1403136
backport: juno
(cherry picked from commit 549be4ba1d84ba749ea79c7a0d1e8953ef9d4cfd)