Tidy up logging

[neechbear]

Logging is currently provided by logrus, but is a little cumbersome. It could most likely be replaced with something simpler.

Logging to syslog and to the local terminal is necessary.

Lets not over-think this.

https://github.com/nokia/gitcache-ssh/blob/master/gitcache-ssh.go#L20

// TODO: Fix logging so that STDERR and Syslog logging can have independent log
//       levels. Ideally, we want to do the following:
//     - Always log everything (up to DEBUG loglevel) to syslog, no matter what.
//     - Change the loglevel filter for messages going to the console (Stderr),
//       independently of the syslog loglevel. By default console messages
//       should have a loglevel of INFO.
//     - When the console (Stderr) isn't an IsTerminal, then a different
//       loglevel and different (simpler) textFormatter should be used for the
//       console logs. This should be independent of the syslog loglevel and
//       textFormatter, which should remain unchanged.
//     - The above will allow all message output to be send through a single
//       log interface, allowing constant detail in syslog, but differing output
//       to the console depending on runtime context.