Improve reading of json from Gerrit API

The PR improves the way Gerrit API result json is queried so that
the test is more robust.

In addition, a potentially failing and unneccessary "visit()" call
was removed.

Lastly, the use of ProcessBuilder was re-worked to improve debugging
for failed command executions.
Shouldn't this block above be in saveNewTestServerConfigIfNone method (called below) instead? This would be so that any potential test calling that method benefits from such careful waiting. Also, the 4-lines-long comment could be removed by means of putting that block in a helper method (in GerritTriggerNewServer) named something like "avoidOccasional404fromNonReadyGerritUI".