Add logging to appliance init

[DanielXiao]

Write logs of ova-webserver to /var/log/fileserver.

VIC Appliance Checklist:

• Up to date with master branch
• Added tests
• Considered impact to upgrade
• Tests passing
• Updated documentation
• Impact assessment checklist

If this is a feature or change to existing functionality, consider areas of impact with the Impact
Assessment Checklist

Fixes #777

[YanzhaoLi]

I think we should also check whether conf.logPath is a directory. Because users may provide a filename path to conf.logPath.

[DanielXiao]

MkdirALL will return Error if it is a file

		if dir.IsDir() {
			return nil
		}
		return &PathError{"mkdir", path, syscall.ENOTDIR}

[YanzhaoLi]

Do we need to handle closing the logFile, or the trace.Logger.out already help us handle it?

[DanielXiao]

We can't close the file descriptor because the running process will write logs to it until it exits.

[wjun]

Since we use journal logs in vic appliance vm already, it seems we do not need to introduce another log file?

[DanielXiao]

@zjs Do you think it is necessary to write logs to a file?

[zjs]

I think #777 is about improving the content of the logs, not changing how they're written (although that may be useful too, as a vSphere Administrator attempting to debug may be more used to finding files in /var/log than checking a journal — but in that case I think we'd want to write them both ways).

For example, I believe we don't currently get good logging around the plugin registration process or failures that can occur there.

[DanielXiao]

@wjun From logging design principle https://github.com/vmware/vic-product/blob/master/installer/docs/DESIGN.md#logging, the init log should be written to a file.