Truncate beginning of S3 log filenames

[kwm4385]

Truncate the beginning of s3 log files to enable reading of useful information.

[ssalinas]

unfortunately the log name will not always be service.log. That is the default for the SingularityExecutor, but it could be anything for s3 logs. I might go more on the route of taking the taskId out of the name (since we are already on the task page) to make a name that is more readable and shorter without having to truncate

[kwm4385]

I was afraid that was the case. This model doesn't include the task id but I should be able to add it in.

[tpetr]

This looks good but to be fully complete, we should surface the s3KeyFormat to the API (https://github.com/HubSpot/Singularity/blob/master/SingularityService/src/main/java/com/hubspot/singularity/config/S3Configuration.java#L28-L34), and strip things off from there.