Don't expose credentials to tempest.conf when
--create-test-accounts is used.
When generating tempest.conf with demo creds, use
--create-accounts-file argument so that the argument
is tested in the gates.
The argument is used only on Devstack.
Tempest concurrency of devstack demo job is reduced to 1
because the minimal accounts file is used.
Change-Id: Id5c90810666d783cf3939086ef27149ef53277f8
Story: 2003016
Task: 23036
c6ec0bdc79