[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.
If its commented, please remove