Postgres and Redis Exporters shouldn't fail if their user account does not exist

In the event either the Redis or Postgres user does not exist locally and the exporters are enabled, the reconfigure step fails. This occurs most commonly when either are running external to Omnibus. Another potential use case we need to consider is MySQL users.

For example, here is a log message for when Postgres is running externally.

52.167.222.89 Generated at 2017-03-16 16:25:47 +0000
52.167.222.89 Chef::Exceptions::UserIDNotFound: directory[/var/log/gitlab/postgres-exporter] (gitlab::postgres-exporter line 23) had an error: Chef::Exceptions::UserIDNotFound: cannot determine user id for 'gitlab-psql', does the user exist on this system?

We need to ensure that the reconfigure step does not fail in this scenario.