slaprunner: Fix bashism in Nginx & Supervisord logrotate entries

kill -SIGXXX works only in bash and not when the shell is e.g. dash, for example:

$ kill -SIGUSR1 1
/bin/sh: 2: kill: Illegal option -S

This way log rotation for slapproxy.log & friends was not properly working.

Fix it by using kill -XXX form which is supported everywhere.