anbox-cloud-tests -generate-config usability improvements

[Samuel Allan]

anbox-cloud-tests -generate-config generates a mix of info text/prompts and yaml formatted data to stdout, where the yaml would usually then be copy/pasted to a file to be tweaked as necessary and used later for the tests.

It would be nice for an easier method of getting this yaml into a file. For example, a `-o <output-file>` flag, or info text to stderr and yaml to stdout to facilitate piping the data to a file.