azure-lb: Be quiet during stop operation

Currently, it logs "kill (<pid>) No such process" to stderr during stops.

A stop operation is expected to run kill -s 0 $pid for a nonexistent
PID, so log that at debug level.

A start or monitor operation's kill -s 0 $pid should always succeed,
so any output is unexpected and an error.

Also remove "local" bashism.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>