Add new inherited methods for kubernetes_mgmt
Add new inherited methods for kubernetes_mgmt

2 new methods are added at last time merge of
Wallaby release in vnflcm_abstract_driver.py.
  * change_external_connectivity_start()
  * change_external_connectivity_end()
(tacker/vnfm/mgmt_drivers/vnflcm_abstract_driver.py#L88)

In kubernetes_mgmt.py,
(sample/mgmt_drivers/kubernetes_mgmt.py)
we inherited VnflcmAbstractDriver thus we also
needed to implements these 2 methods here.

Due to we don't have these interfaces here, I updated
empty methods for clear runtime error.

Change-Id: I9f918ceff9882da1bff4b1cc953602a3b84aec0c

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