[FLINK-20510][conf][log4j] Enable monitorInterval by default

[zentol]

This PR sets the log4j monitorInterval to 30 seconds by default, allowing the configuration to be changed at runtime.

Note: I tried to do the same for logback, but it just kinda broke down and wiped the existing log file.

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[tillrohrmann]

Thanks for creating this PR @zentol. LGTM. +1 for merging. I guess we should also document this in the logging documentation so that people know that changing configuration only works when using log4j2.