Redrive rate limited API calls.

Transparently redrive calls throttled by nova rate limiting, with
an intra-retry delay governed by the Retry-After response header.
Invocations are retried at most twice.

This avoids spurious test failures when the target nova installation
is using the default rate limits, or explicitly configured limits too
low for the cadence of tempest test cases.

Change-Id: I6c701b24f59998a98e6f515a2861a458470ee40c
1 file changed