config: config file path from STARCLUSTER_CONFIG env. variable
config: get AWS credentials/region using same env. variables as EC2 command-line tools
Merge branch "feat-ec2tools-env" (config: get AWS credentials/region using same env vars as EC2 cmdline tools)
config: fixed crashes when $EC2_URL env var is not set
Merge branch "feat-ec2tools-env" (fix crashes when $EC2_URL env var not set)