Add spot_allocation_strategy variable with default price-capacity-opt…

[brightshri]

…imized

• Add spot_allocation_strategy variable to _variables.tf with default value 'price-capacity-optimized'
• Update asg.tf to use the new variable in instances_distribution block
• Remove spot_instance_pools parameter as it's not compatible with price-capacity-optimized
• Addresses AWS deprecation of 'lowest-price' spot allocation strategy

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

Types of changes

What types of changes does your code introduce to <repo_name>? Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [ ] I have read the CONTRIBUTING.md doc.
• [ ] I have added necessary documentation (if appropriate).
• [ ] Any dependent changes have been merged and published in downstream modules.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...