[WFLY-1877]: Make KeepAliveTimeAttributeDefinition deal with both a simple 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.

Jira: https://issues.jboss.org/browse/WFLY-1877
So I'll have to update also he PR for WFLY-1878 which is kind of similar.