MCO-456 expose target collective to discovery plugins

With this commit we expose the target collective to the discovery
plugins, by merging a key "collective" into the filter hash as it
is passed to the `discover` method of the discovery plugin.

It's expected to be useful in the mongodb discovery filter per this
mailing list thread:
https://groups.google.com/forum/#!msg/mcollective-users/wMk2ms5yp5M/11OEJT6MxewJ