osstest: stop anacron service

osstest: stop anacron service

Just disabling cron in rc.d is not enough. There's also anacron which
will get invoked during startup, and since apt-compat has a delay of
up to 30min it can be picked up by the leak detector if the test
finishes fast enough:

LEAKED [process 14563 sleep] process: root     14563 14556  0 07:49 ?        00:00:00 sleep 1163
LEAKED [process 14550 /bin/sh] process: root     14550  2264  0 07:49 ?        00:00:00 /bin/sh -c run-parts --report /etc/cron.daily
LEAKED [process 14551 run-parts] process: root     14551 14550  0 07:49 ?        00:00:00 run-parts --report /etc/cron.daily
LEAKED [process 14556 /bin/sh] process: root     14556 14551  0 07:49 ?        00:00:00 /bin/sh /etc/cron.daily/apt-compat

From:

http://logs.test-lab.xenproject.org/osstest/logs/169015

To prevent this disable anacron like it's done for cron.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
Changes since v1:
 - Don't fail if the service is not present.