[WFCORE-39] : Make KeepAliveTimeAttributeDefinition deal with both a sim...

...ple long and a complex attribute.

Transforming KeepAliveTimeAttributeDefinition in Model from a complex Object to a Long.
All keep-alive now are expressed as long in milliseconds.
This will affects all subsystems using the "threads" subsystem in wildfly which will be treated in another PR once this one is integrated and released.