This patch provides Tacker documents for change current vnf package
in V2-API. The specification about new API is added to API Reference.
Also, change current vnf package operation is added to
Tacker Use Cases Guide and CLI Reference.
This patch also adds change current vnf package restrictions to
error_handling user guide.
Implements: blueprint upgrade-vnf-package
Change-Id: I3f18bbab17c67e717dc0a9d36e92f812a40862e5
f7e241eca2