Add ".log" suffix for user-stdout/user-stderr/user-all log files

[Binyang2014]

Add ".log" suffix for user-stdout/user-stderr/user-all log files
Fix: #5243

[coveralls]

Coverage remained the same at 33.91% when pulling 4aab288 on binyli/log-suffix into 30f21b3 on master.

[Gerhut]

Why not directly add .log to the actual url? Avoid breaking change?

[Binyang2014]

Why not directly add .log to the actual url? Avoid breaking change?

Because it's easier to implement and do not need to care about how log is stored.
The log url we returned is: https://pai-master/log-manager/..../api/v1/logs/user-stdout?...
The real log is stored at /var/pai/log/.../user-stdout/current
And we have some url such as https://pai-master/log-manager/..../api/v1/logs/user-stdout.1?... for rotated log.
The real log is stored at /var/pai/log/.../user-stdout/@....s

If we change url, we need to extract the real log folder before try to get the content. Using this way to avoid string manipulate