SqlSetup: When installing R Services (In-Database) it should be able to use MRCACHEDIRECTORY argument

Details of the scenario you tried and the problem that is occurring:
When installing R Services (In-Database) it should b able to use MRCACHEDIRECTORY argument for doing offline install.

https://docs.microsoft.com/en-us/sql/database-engine/install-windows/install-sql-server-2016-from-the-command-prompt

As @kewalaka points out in issue #340, there need to be a mechanism to copy the files to the target node. Maybe we can leverage the already present SourceCredentials, which when set will copy the media to the target nod in a temporary location.
So if MRCACHEDIRECTORY is set to the source UNC path, and if SourceCredential is assigned credentials, the source UNC path is copied to the target node.

There s some more information in the section '
Scenario 4: Unattended setup of SQL Server 2016 RTM' in the article Do it right! Deploying SQL Server R Services on computers without Internet access.

The DSC configuration that is using the resource (as detailed as possible):
n/a

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
n/a

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:
n/a

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Dev

Start at the SqlSetup resource and trace how MRCACHEDIRECTORY and SourceCredentials are handled during installation. Reproduce the offline R Services installation scenario described in the linked SQL Server documentation and verify that the source media reaches the target node and setup completes without Internet access.