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

@adietish, @maxandersen : please review and leave comments for any suggestions/required changes.
Your sentence seems right. The restriction I was reporting is basically only saying that you can't have a public key uploaded twice using different names. OpenShift only accepts unique keys in name and in content. 
The default usecase is 1) you upload your key for the first time 2) you re-upload your key after having removed it.
I have for instance a specific key for OpenShift, I dont use the same key for let's say github, OpenShift etc.