Fixing issue 1284 and --no-ssh-password paramater handling logic

Reference: 
https://github.com/Azure/azure-sdk-tools-xplat/issues/1284
https://github.com/Azure/azure-sdk-tools-xplat/issues/658
https://github.com/Azure/azure-sdk-tools-xplat/issues/1003
https://github.com/Azure/azure-sdk-tools-xplat/issues/1041
This is the same code as before (just moved up): to confirm, this is used as a string so that a simple check (`if (config.disableSshPasswordAuthentication) { /* then something */ }`) works?

Just want to make sure string is the right thing.