AbstractKapuaConfigurableResourceLimitedService throws npe if maxNumberChildEntities is not present

[riccardomodanese]

Describe the bug
AbstractKapuaConfigurableResourceLimitedService throws npe if maxNumberChildEntities is not present in configuration map due to implicit cast to primitive.
Should be investigated if other implicit casts can throw npe.

To Reproduce
none

Expected behavior
no exception thrown

Screenshots
none

Version of Kapua
1.2.2

Type of deployment
[x] Local Vagrant deployment
[x] Docker
[x] Openshift (in its variants)
[x] Others

Main component affected
[x] Console (in case of console please report info on which browser you encountered the problem)
[x] REST API
[x] Message Broker
[ ] - Others

Additional context
none