Update Nexpose::Connection#list_discovery_connections for mobile and DHCP

[erran-r7]

Currently discovery connections are shaped around AWS and vSphere connections. The DiscoveryConnection class and Connection#list_discovery_connections method should be updated to only return the relevant values for each connection type.

DHCP connections over Microsoft DHCP and a directory watcher for example do not include a port value, but do include an engine ID (which no other discovery connections use today).

[gschneider-r7]

Hmm looking into this, I think there is something fishy about the XML response currently. We're showing an engine ID for non-DHCP connections, which as you said is not used that way. This seems like it would not validate against the 1.2 API schema. Edit: The schema has been updated for the optional fields, but I feel like there would still be problems for some other clients consuming it.

[gschneider-r7]

This is not currently possible to implement in the gem. New API endpoints should be exposed from Nexpose first.