Labs: provide saner nscd defaults

Labs has different use cases, and several assumptions about
resolving can be made that impact the best settings for nscd:

- users are very rarely removed
- groups are slightly more mutable
- DNS entries tend to have long lifetimes

Accordingly, some of the caching values have been tweaked to
reflect that reality and greatly reduce the load on the LDAP and
DNS services, which are not all that robust under load.

Note that this turns host caching back on, despite it being
turned off by default in recent Ubuntu releases because of a
known issue:

https://sourceware.org/bugzilla/show_bug.cgi?id=4428

This bug has essentially no impact in Labs because the TTL of
Openstack-created entries in dnsmasq is not normally tunable.

Also, the worst case scenario is that some library calls returned
cached values with a lifetime of the greater of the DNS TTL and
that of the cache (which is incorrect, but mostly harmless since
we keep our cache lifetime at 1h).  In practice, this means that
/if/ we have the TTL of a hostname under one hour, /and/ it
changes while cached, it may persist for up to 1h more than it
should or until the cache is manually invalidated.  This seems
to be acceptable.

Bug: 70076
Change-Id: Ic41a10d2fbafabdd50c7db49580ace717499c829
Merge "Labs: provide saner nscd defaults" into production