Updated get_default_config() to check if username and/or password are present on the command line

Username and password can be set in the configuration yaml file, and can be overwritten using the command line. Without this fix, it would always use the command line username/password combination, which would make the config file options useless.
Merge pull request #4 from Recras/master

Update get_default_config() to check if user/pswd are set on command line