This commit adds python logging to test_framework.py. By default this
will output all log levels (DEBUG-INFO-WARNING-ERROR-CRITICAL) to a
test_framework.log file in the temporary test directory, and higher
level logs (WARNING-ERROR-CRITICAL) to the console. The level of logging
to the console can be controlled by a new log-level parameter.
This should have no interaction with the existing trace-rpc parameter.