afmongodb: Mark host() and port() obsolete

The host() and port() options should have been marked obsolete a while
ago, fix that now, and suggest using servers() instead. This fixes #92.

Reported-by: Andras Mitzki <micek@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #104 from algernon/f/mongodb/obsolete-keys

afmongodb: Mark host() and port() obsolete