Get-DcnImage: Use the stored credentials when querying the db if none are supplied on the command line.

[jonstace]

When working in an environment that doesn't have Windows auth set up, so SQL credentials are required, Get-DcnImage doesn't use the SQL user credentials saved during the call to Set-DcnConfiguration. If you call Get-DcnImage without any other parameters, the current Windows user is used to authenticate, and if there isn't a domain/trusts set up, authentication will fail. This then cascades through to other commands that call Get-DcnImage without parameters.