Implementing a smart version of the rescanExisting flag

Now, the old behavior has been resurrected, and a new one has been
added:

rescanExisting=false, means only add/remove nodes from the DB skipping
the scan phase.

rescanExisting=true (default), perform all the DB updates and execute
the scan phase.

rescanExisting=dbonly, perform all the DB updates skipping the scan
phase.
Merge pull request #129 from OpenNMS/jira/NMS-6860-1.12

jira/NMS-6860 for OpenNMS 1.12.
Improving logging for rescanExisting and fixing JUnit tests.
[bamboo] Automated branch merge (from 1.12:b039bce4397a7270a7d7b74281f675517363366f)
Merge remote-tracking branch 'origin/jira/NMS-6759-1.14' into 1.14.0
Adding support for rescanExisting to provision.pl
[bamboo] Automated branch merge (from rc/stable/1.14.0:02bba5ccb39306dcd62f3b9fe7678cb623ea296d)