Add some more checks to engine creation

Add a field called enabled in the engine cfg file that keeps track of
files. This can be used to keep track of stopped engines, and
monitored using watchdog/similar. Also added some more checks at
engine creation time.

Partially Closes-bug:1309406
Change-Id: I1c365c2c438e6ed0a44413e1d09c69d3fab7ab7b