Make startup script work on Ubuntu

Empty value for PIDDIR_OWNER causes code on line 51 and 55 to fail. This resulted in the following output at startup time (in /var/log/boot.log):
install: invalid user `-g'
 * PID directory was not found and created
Error starting /usr/local/bin/newrelic-plugin-agent: Cannot write to specified p
id file path /var/run/newrelic/newrelic-plugin-agent.pid