Add cnf-conformance in functest-kubernetes-cnf

Add cnf-conformance in functest-kubernetes-cnf

It selects cnf-conformance v0.7.2-beta1 (current latest release) and
the default CNF as proposed by [1].

The end user is free to override the default yml files on purpose (e.g.
docker volumes). Tag (default: all) could be modified too via
testcases.yaml

[1] https://hackmd.io/@vulk/SkY54QnsU

Change-Id: Iec2a830c441e523e067f6eb114ecb205c49c6b93
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 120daffd8d6d4be727d2ba52a76dbd8eacfe2628)