Allow verifier query set use different username and password

presto-verifier is an excellent project to make sure any changes on the plugin doesn't break anything.

But in our test cases, we've used tens of different username and password at the query sets. At the first time, we tried to create different suite per credential, the numbers of suite kept growing and it's getting hard to manage and merge the verification result. 

So if the query set allows to store username and password and use them, we could run the verifier without changing every time or creating tens configuration files. Also it should be allowed to override the stored value as the verifier is already doing on the catalog and schema.