Refactor healthcheck-k8s.py

Refactor healthcheck-k8s.py

Error reporting was improved by returning actual
error message from subprocess call to 'kubectl' command.

Code readability was improved by defining a dictionary
of endpoint names to check and their IPs.

Unsecure 'shell=True' property in Popen constructor for
kubectl command in SDNC DB was removed and command string
itself sanitized.

Overall code readability was improved by reusing common
commands in a loop.

Change-Id: I19f8f71e27196bb55a9be3d58cd0885ceba3af0c
Issue-ID: TEST-213
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>