Fix OPENDJ-737 - OpenDJ Administration Connector KeyStore Pin File must be defined and non empty
Review CR-2344

NPE occurs in AdministrationConnector#createSelfSignedCertificateIfNeeded method.
To avoid this, now attempt to create certificate only if initial setup has not been changed. 

* AdministrationConnector.java : 
  - completed method createSelfSignedCertificateIfNeeded to add more checks to detect when initial setup has changed
  - added method hasDefaultConfigChanged to isolate the check

* No unit test added, functional test to be added