Changes

fake_trx: Allow setting sched RR priority for clckgen thread

With this patch python is still too slow sometimes, with frecuent
overruns in the range 50-2400 microsecs. Still, with higher prio we
should hopefully see less cases where the process is being delayed by a
much higher amount, which may trigger a "no clock" error from
osmo-bts-trx.

Change-Id: I3c7f36c17fc7b9cae1023b4a0ed37f983dc286dc