Update Add-on credentials.md for NodeJS guide, for exo-mirror

Minor fix discovered by exoscale where parameter HOST and USER should be substituted to HOSTNAME and USERNAME, for exo-mirror branch
Merge pull request #135 from cloudControl/nodeguide_creds_fix_from_exomirror_branch

Update Add-on credentials.md for NodeJS guide, for exo-mirror
Merge remote-tracking branch 'cc/exo_mirror'