Disable slow_digest job on coredb s[1-7]

Saw pt-query-digest (perl) recently at 100% CPU on a slave under load
and hitting max_connections. Have also previously seen it fighting with
puppet. Whether as symptom or a cause, send_query_digest.sh script needs
to be adjusted to:

- Check for a concurrent puppet run
- Check for CPU usage or general load above a certain point

Change-Id: Ice3d8924c6db75ee66f27a23cc1415074935fac6