zkfarmer: default log level to INFO for join/export

By default, the logging level is WARN which is too low for a
daemon. When using `join` or `export` commands, we default to
INFO. Also, `-v` will increase the log level while `-q` will set
logging level to WARN even for `join` and `export`.
Merge pull request #21 from vincentbernat/fix/default-logging

zkfarmer: default log level to INFO for join/export