Allow for custom log formats to be defined in stacker.yaml

[russellballestrini]

At Remind we run stacker on a large portion of our stacks in stage and production concurrently using continuous delivery. This means our logs are interlaced between stage and prod and are hard to use.

Also since we use continuous delivery we want to put the deployment_id or git commit hash on every log line so that we can aggregate them all up with our log parsing and viewing tools.

This PR makes it possible to set a custom log format from stacker.yaml.

modified:   .gitignore
modified:   stacker/commands/stacker/__init__.py
modified:   stacker/commands/stacker/base.py
modified:   stacker/config/__init__.py
modified:   stacker/logger/__init__.py
new file:   stacker/tests/fixtures/not-basic.env
new file:   stacker/tests/fixtures/vpc-custom-log-format-info.yaml
modified:   stacker/tests/test_stacker.py