[improvement] : use fixed retryAfter instead of exponential backoff

[rahulait]

What this PR does / why we need it:
We would like to use fixed backoff than exponential as exponential starts with microseconds and makes 10+ requests before it reaches 10 sec or higher backoff time. Since POST calls to /v4/linode/instances have low limits, we would like to make sure we don't make too many POST calls in case of an error due to exponential backoff. With this fix, we are now retrying at a fixed interval. This also affects other calls which use the same method to calculate how soon it should retry, but having a fixed interval for them is also fine.
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests