Add option to tune Trillian's log verbosity
It is a bit confusing how the flag names and values interact. But from trial-and-error, it seems the two Trillian settings result in "don't give me any logging on stderr unless it concerns LEVEL or higher".
This commit sets WARNING as the default.
Related to sigsum/core/log-go#87 (closed).
Edited by Rasmus Dahlberg