Add log.file.path to capture the log file path, following Filebeat convention.

[webmat]

A few points to note about this PR:

• This is not nesting all of file.* at log.file.*, this is adding a single field only: log.file.path. If I remember correctly, it's the first time we do this.
  • If there's a need for more file.* fields we could add them, but so far that need has not arisen.
• This approach also made it possible to tailor the field description to this specific use case.
• Where file.path has a text multi-field which may be useful for threat hunting in arbitrary file paths, log.file.path does not. The thinking is that these log files are controlled by operators and aren't "user data" in which we're likely to do threat hunting. This is something we can add later if needed.
• This PR also adjusts the field description for log.origin.file.name to disambiguate: log.origin.file.* are fields to capture source code file details, in application logging use cases.

Closes #770

[webmat]

FYI @roncohen @ruflin

[dainperkins]

looks good