tools: don't rely on Connection.isValid()

Patch 9f856a6234566a2ab648ef580fa102192ec43b10 refactored
StandaloneDataSource.checkConnection() to use Connection.isValid() in
order to avoid a FindBugs warning on an unused function result.

Apparently, this approach is problematic since the PostgreSQL JDBC
driver we're using does not implement isValid().

This patch reverts 9f856a6234566a2ab648ef580fa102192ec43b10 and proposes
a different way to avoid the FindBugs warning.

Change-Id: Ic4f1fe6a9981c6fe8764a803eda4de5d51cc2b6e
Signed-off-by: Allon Mureinik <amureini@redhat.com>