Added support for specifying logging level via flag.

minloglevel flag is passed as an argument to the command line and it
configures the level of logging to stderr and file. It can take INFO,
WARNING, ERROR or FATAL values. All log messages at or above the
configured log level will be printed.

If 'quiet' flag is also passed to the command line, minloglevel will
affect just the logs from file (if configured by log_dir flag)

Review: https://reviews.apache.org/r/19357