create -t aws-ec2 should ...

[yarikoptic]

• log some details at the DEBUG level (e.g. that it is about to start an instance on ami blah in zone bleh)
• issue a warning if no credentials were found. IIRC (I might be wrong) boto3 might try "Anonymous" access first, that is where providing empty could make sense so we shouldn't just crash. Ideally a warning should provide instructions on where to store those credentials. apparently "standard" location for configs on mac is not ~/.config/reproman/ and it took direct interaction with appdirs to figure out what it really is