cat-log: infer --force-remote if log file not present locally

[oliver-sanders]

• cylc cat-log <job-id> will cat/tail the requested log file for the specified job.
• If the job is running, it will go to the remote filesystem to do this.

Issues:

1. There is a lag between a job finishing and job log retrieval being performed.
2. Sometimes not all log files are retrieved due to files exceeding the maximum configured size, or possibly retrieval errors.

Solution:

• If the user requests a file which is not present on the local filesystem, fallback to the remote filesystem.

Note:

• This helps to reduce the caveats of removing the --force-remote option from the UI Server's use of cylc cat-log --mode=list-dir: Allow configuration to tell cat-log to fetch logs from (local) disk rather than remote host cylc-uiserver#506

[oliver-sanders]

(optimistically tagged 8.6.0, but this should not delay release) (bumped to 8.7.0, pull this back to 8.6.0 if it gets merged in time)