Updates to linux-use-ssh-key

- Added and fixed screenshot in step 7 of "Create a PPK for Putty"
- Added new information about translating an openssh key to a x509 cert
for use with Azure
- Misc. edits for clarity
- Updated "authors" tag
Merge pull request #1770 from szarkos/linux-use-ssh-key

Updates to linux-use-ssh-key