CLOUDSTACK-883. DOC. How to integrate your 3rd-party plugin with the UI. Contains brief third-party plugin code tutorial.
CLOUDSTACK-4528: [Vmware] new mapping vmware datacenter cloudstack zone - Virtual Router host migration across cluster FAIL

Changes:
- listHosts within same cluster for migration of system and router VMs
CLOUDSTACK-4528 [Vmware] new mapping vmware datacenter cloudstack zone - Virtual Router host migration across cluster FAIL

Change:
- Also add a check in migrateSystemVM API to check that source and destination host are in the same cluster
CLOUDSTACK-883. DOC. Small fix to comment in UI plugins section.
CLOUDSTACK-883. DOC. Fix license header in UI plugins section.