On Labs, provision an Apache config dir that is not managed by Puppet

In production, we try to be punctilious about having Puppet manage
system state, and thus it's reasonable to purge Apache site configs
that have not been declared via Puppet. But on Labs we want to allow
users to manage configuration files locally if they so choose,
without having Puppet clobber them. So provision a
/etc/apache2/sites-local directory for Apache to recurse into during
initialization, but don't manage its contents.

Bug: 66751
Change-Id: Id47a01d49e9bbdca59a86b0da16ff42da4aed8ea
Merge "On Labs, provision an Apache config dir that is not managed by Puppet" into production