Bug 1127410 - WARN [ResourceDiscoveryComponent.invoker.daemon-10] (org.rhq.plugins.postgres.PostgresDiscoveryComponent)- PostgreSQL data directory (/var/lib/pgsql/data) does not exist or is not readable

Log at debug level in discovery code (which is called often)
Log problems with the config file only once when the resource component
is started

Also, some code cleanup
(cherry picked from commit c2b6f256d0f045e8cdb741e78e3c79254c203af8)

Signed-off-by: Jay Shaughnessy <jshaughn@redhat.com>