burnin: rename instances to same name

this (a) allows us to test renaming in burnin even if we don't have an
extra instance name and (b) is a valid operation, as it allows to "fix"
an instance who went name-astray

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>