Fix conflicts with _member_ role creation

Historically, the installation guide manually created the
internal _member_ role to resolve issues with horizon.
However, keystone will preferably create the _member_ role
automatically if the 'user-create' command includes the
'--tenant' option.

Change-Id: I1a67db2b6aa6a8e2bfd76cc80db1fb09fa353986
Closes-Bug: #1403136
backport: juno
(cherry picked from commit 14e6c86d5a457dbbb90690d55655a4532919255a)