Remove logentries-system-stats file creation,add try catch when accessing stats

This remove the logentries-system-stats.log file creation in the install script.

Also added a try catch for when we try to access the system metrics dictionary in case such values don't exist ([See SmartOS issue](https://github.com/logentries/le/issues/60)). For the moment if there is an exception when trying to access one of these objects then the stats will not be sent.
Other option is to use data.get( .., ..) or test one field in data like if 'cu' in data that would be cleaner