Fix error when dataset credential is expired
We use stdout to the get the credential from managa_dataset, but a bunch of stderr junk ends up in there when the credential is expired. Need to clean that up
Your topology source contains invalid values. Malformed arguments: ***
verifygenicred: /tmp/AaKp6Bx0wL does not conform to schema
/usr/testbed/lib/protogeni/security/credential.xsd