Add ability to configure amonagent logging to custom file.

[eliezerlp]

Currently amonagent seems to log to stderr using the default configuration of https://github.com/sirupsen/logrus/. When run as a systemd service this results in the logs going to syslog.

Our desire is to be able to log to a separate file instead of to syslog in order to be able to set a custom logrotate config for it.

It makes sense that the would be a on optional configuration in amon.conf that allows you to define where the log file would be stored.