[BUGFIX] Fixed current extensionscope for fetching viewconfiguration

FluxService::getViewConfigurationForExtensionName() should fetch
TypoScriptSubConfiguration using the extension key, not extension
names like "Vendor.ExtName"
Merge pull request #598 from higidi/bugfix/extensionscope-for-fetching-viewconfiguration

[BUGFIX] Fixed current extensionscope for fetching viewconfiguration