Fix log level settings

Fix two bugs that were making the log writer always output at debug level:
* Config::setStr() returned an empty string rather than the expected
  default value when no value is present in the environment.
* Monolog expects log levels to be specified as integers corresponding
  to the constants in Monolog\Logger rather than strings as used in
  Psr\Log\LogLevel.

Bug: 59725
Change-Id: Idcac0d6548f5f68335c682300db2171284aaacea