Allows passing existing keys and salts vs generating afresh

In a multi-server environment, if you want a consistent configuration without having to patch the automatically generated wp-config.php, there should be a way to pass the common keys and salts.
Switched from --keys-and-salts="..." option to --skip-salts based on @scribu and @danielbachhuber proposal
Don't include the comments, which would be irrelevant without the salts.

Based on @scribu's idea.
Added tests for the --skip-salts parameter
Fixed a typoe in --skip-salts synopsis
Merge pull request #696 from nikolay/patch-1
fix and simplify the --skip-salts test. see #696