Cover the tempest user in select_mirror

* devstack-vm-gate-wrap.sh: Add the same mirror configuration for
the tempest account as is set for root, jenkins and stack users.

Closes-Bug: #1260371
Change-Id: I9be4bdfb9d8d1966e8ddfbeaec75c98e475026e6
Merge "Cover the tempest user in select_mirror"