Use `member` instead of `Member`

Keystone now provides a set of default roles in addition to `admin`
by default [0]. This is done during the `keystone-manage bootstrap`
process.

This change aligns the `Member` role override from devstack with the
`member` role provided from keystone.

[0] https://review.openstack.org/#/c/572243/

Change-Id: I3da3530aa73a8a1500116bcefdcba7b947d5e05e
Closes-Bug: 1777359
2 files changed