FIX - petascope should use rasdaman_user to query when authentication_type is empty

[Bang Pham Huu]

If authentication_type is empty and rasdaman_user is set with a value in petascope.properties then it should use that user to request for non-updating requests (e.g. GetCapabilities, GetCoverage,…).
It should not use rasadmin to request for these read requests which will return more coverages than it should be for the unauthenticated user.