[#1539] Use explicit message when generating SSH key without a comment.

Problem description
===============
The message that is displayed when generating a SSH key without a comment should be more user friendly.

Changes description
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A new message will be added if no comment is specified for the SSH key in the command line options.

How to try and test the changes
========================
reviewers @adiroiban @alibotean 

Look at the code and run the tests.