labs: reduce acct archiving retention

In our standard packages we install 'acct' which does user and process
accounting (see commands 'ac' and 'lastcomm').  The log are held in
/var/log/account which, on labs is a 2GB partition that ends up filling
quite rapidly on busy instances.

On the beta cluster deployment-bastion instance, it takes up to 500MB or
a quarter of the /var instance.  Reading the SAL I noticed we keep
cleaning up manually to reclaim disk space.

The files are rotated via a daily cron using the 'savelog' command. It
is passed $ACCT_LOGGING which default to 30 days.  Reduce it on all labs
instance to 7 days.

See also
https://wikitech.wikimedia.org/wiki/Incident_documentation/20140910-BetaCluster

Bug: 69604
Change-Id: Iae6adb6e5783e6ddbcba11779d204aff8e76f9b9