pkg/asset/installconfig/aws: Ask for AWS access key and secret
pkg/asset/installconfig/aws: Ask for AWS access key and secret

For folks who forgot to setup ~/.aws/credentials before calling the
installer.  Now we'll give them a nice prompt and a link into the AWS
docs instead of dying.  I'm also saving any provided creds into the
usual location for later AWS access during this and future installer
runs.

I'm constructing my own provider chain so I can drop the
RemoteCredProvider which github.com/aws/aws-sdk-go/aws/defaults wants
to add.  I'm not entirely clear on what that's about (maybe [1]?), but
it was hanging occasionally in my local testing.

[1]: https://aws.amazon.com/blogs/security/how-to-eliminate-the-need-for-hardcoded-aws-credentials-in-devices-by-using-the-aws-iot-credentials-provider/

derekhiggins/installer,derekhiggins/installer