Remove built-in event driver

[kars7e]

• Removes the logic and every mention of built-in event driver
• vCenter event driver has been moved to https://github.com/dispatchframework/dispatch-events-vcenter
• Fixes a bug in libkv-based entity store where shared prefix could result in entities of a different type to be returned when listing.

Closes #558

TESTING:

Tested locally running:

$ dispatch create eventdrivertype vcenter dispatchframework/dispatch-events-vcenter
$ dispatch create secret vcenter vcenter_secret.json
$ dispatch create event-driver vcenter --secret vcenter