Add vnf definition APIs

Add vnf definition APIs

Adding APIs for POST, GET, LIST (implemented via GET)
and DELETE commands on /v1/vnfd base for creating,
getting, listing and deleting VNF Definitions.
P2: Added unit tests for vnfdhandler.go
P3: Add unit tests for serialize and deserialize
P4: Integrating review comments
P5: Added customizable mocking for vnfdhandler_test
P6: Added customizablt mocking for vnfd_test
    Note that this will soon need to be updated once
    the db changes go through in patch 71090

Issue-ID: MULTICLOUD-393
Change-Id: Id509bed370ab3bdc572c6ead22324c1ee3dbf82d
Signed-off-by: Kiran Kamineni <kiran.k.kamineni@intel.com>
Signed-off-by: Victor Morales <victor.morales@intel.com>