Deprecate API_RATE_LIMIT setting for Tempest runs

The default value for api_rate_limit in nova.conf has been False since
Havana per commit cfdc59860ef1482e3b064f90f719ec9e484590ce.

Devstack commit f617a2ddddf7d8f759ff3ca5d66a133b124eeb42 removes the
API_RATE_LIMIT variable.

Since we're not going to backport the devstack change to the stable
branches, this patch adds a TODO in the script to remove the variable
when stable/juno is our oldest supported branch.

Change-Id: I21dbd72f9959a4e3d62c36831df03b351a51172e
Merge "Deprecate API_RATE_LIMIT setting for Tempest runs"