TOOLSDOC-510: Added OS How To 'Configure SSH Keys'

@adietish, @maxandersen : please review and leave comments for any suggestions/required changes.
@adietish : 

For the usecase for using existing keys I have now got "This option may be applicable if you have removed a SSH key pair from your OpenShift account, still have the generated key files available on your system and want to reassociate them with your OpenShift account."

BUT is there a better use case for the 'Add existing keys' function?