Adds an alternate user to Keystone for Tempest
Tempest has a number of tests that are skipped if
the compute.alt_username is the same as compute.username
or None. Here, we modify files/keystone_data.sh to add
an additional regular user called alt_demo if Tempest
is enabled in stackrc. We also make corresponding changes
to the tools/configure_tempest.sh script to make use
of this alternate user credential
Change-Id: I551f3b378f843c62fffcf6effa916056708d54d3
b297d2d0a8