Reduce excessive log messages from authorized_keys table implementation

[nabilschear]

This table implementation produces a lot of unneeded error messages for files that don't exist when cross joining against all users on a system:

$ osqueryi "select * from authorized_keys cross join users"
W0922 13:52:54.039000 354004480 glog_logger.cpp:34] Cannot open file for reading: /Users/nabil/.ssh/authorized_keys2

This PR introduces a check to see if the file exists before attempting to open it. If there's no file, it will continue silently.

[nabilschear]

@theopolis this one is an easy one

[zwass]

This totally makes sense. I had no idea we were just assuming a file existed at the expected path.