[psa] Use spot instances for non-production ASGs

Hi engineers,
Let's use AWS Spot Instances, starting from the non-production environments!

What is spot instance? Why should we use it?
Read our EC2 Spot Instances wiki.

What do we need to do?

for tvlk-dev, I'll enable AutoSpotting to all tvlk-dev's ASGs on April 29, 2020.
for dedicated accounts (i.e. tvlk-<PD>-dev & tvlk-<PD>-stg), please follow @micky's guide on using spot with ASG mixed instance policy

168 tvlk-dev ASGs have been converted to spot instances (thanks!); these clusters are now running with 69% savings compared to on-demand EC2.

Using spot instances in production will give us even more savings. However, we need to understand the risk. Check the "should I use spot instances ?" section in the wiki as a starting point. Your inputs and contributions are very welcome. We can also discuss in #asg_spot_instances.

Let's use this moment to improve our application flexibility and fault tolerance, so we can later run production workload more cost-efficiently with spot instances!