Add toggleable yappi profiling to zuul

Add a bit more functionality to the SIGUSR2 handler in zuul. When that
handler is run toggle the state of the yappi profiler and when we turn
the profiler off log the profiling data. Do this in addition to the
normal thread dumps.

Change-Id: I127cfb924f0576072d26bde0b4204f8636195d97
Merge "Add toggleable yappi profiling to zuul"