[Please merge this one] Azure Data Factory Dec release updates for Sprint 42

Compared to the previous PR (https://github.com/Azure/azure-powershell/pull/27) sent by AKhadm, this one uses the "Credential" instead of "WindowsCredential" as the parameter name according to Ogail's suggestion.

The changes included in this PR:
a) update the datafactories MAML nugget package to version 0.12.0-preview
b) Remove -location parameter from New-azuredatafactorygateway cmdlet
c) Add -Credential parameter in New-azureDataFactoryEncryptValue to encrypt windows authentication connection string encryption.
d) Update the related scenario test cases by recording new sessions.
e) Update the related help contents reviewed by UE.