BS deployment: get AWS credentials from aws configure

[vbaranov]

Problem: for now for every deployment I need to put AWS credentials to ./group_vars/all.yml. Meanwhile, I have preconfigured credentials with aws configure

Solution: if there are no aws_access_key and aws_secret_key filled in the config, authentify from aws configure credentials if it possible.

[ArseniiPetrovich]

This one can be tricky, but I think I find the way.
What I propose is to use default(omit) to suppress variables if they are not set.