SRU will be triggered twice in the autostart file

For the command we use for starting SRU test in the autostart file, it will trigger the test twice:

look into the command we use:
sh -c "pidof systemd && { sudo systemctl start checkbox-ci-installed-notifier.service; /usr/bin/checkbox sru --check-config >/tmp/checkbox.log 2>&1; sudo systemctl start checkbox-ci-mailer.service; } || { sudo initctl emit checkbox-sru-started; /usr/bin/checkbox sru --check-config >/tmp/checkbox.log 2>&1; sudo initctl emit checkbox-sru-finished; }"

The problem is that on a SRU system, it doesn't have the checkbox-ci-mailer package installed.

Let's break down what happened on a 15.04 system:
1. it will satisfy the "pidof systemd && {$first_part}" command, run SRU once with $first_part command
2. since it failed with no checkbox-ci-mailer package installed in $first_part command, it will not return 0 for step 1
3. as the return value is not 0, the second part (command after ||) will be triggered, which causing SRU to be triggered twice.

A possible solution is to add another if statement for checking the existence of the checkbox-ci-mailer package, call the mailer service if it's installed.