feat: cancel spot requests

[kayman-mk]

Description

Whenever a spot executor is created a spot request is created. Occasionally there might be open spot requests which have not been fulfilled. These spot requests have to be deleted as soon as the agent terminates as no executors are needed.

Challenge: Make sure that spot requests are deleted only which belong to our module. Unfortunately the GitLab Runner does not tag them. We use the following logic for determinination:

• state is open or active
• they are associated to a SSH key which name starts with runner
• they have the var.environment/var.overrides['name_docker_machine'] somewhere in the name

Solves part of #623
Closes #493

Migrations required

No.

Verification

• kill an agent manually and check the Cloudwatch logs. The function should log Removing open spot requests for environment and print either Spot requests deleted or No open spot requests found. There are no errors in the log.

[npalm]

In general looks good not the time right now to test. Would suggest the following

• Add a module readme to describe shortly what the lambda does
• Add a backlog itme to add unti test.

[kayman-mk]

Oh yes, missed to update the README.md. It's now done.

Issue for unit tests created (#721)

[kayman-mk]

merge conflicts fixed