Pull AWS credentials from ENV if available

Use AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables
for AWS credentials if they are not present in the ec2.yml config.
Merge branch 'pull_config_from_env' of github.com:fixlr/capify-ec2 into fixlr-pull_config_from_env

Conflicts:
	lib/capify-ec2.rb