Build arm* functest-vnf containers via travis-ci

Build arm* functest-vnf containers via travis-ci

It doesn't build juju deps if arm* as they can't be cross-compiled.
Then that deps won't be built in releng on native arm64 hosts
It's worth mentioning that containers have never been executed in
OPNFV gates.

That change allows running all other VNFs on Raspberry PI and
building containers without arm build servers.

Change-Id: I836a72f049e7f0e6ae704f5b829914692d5b2c09
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 0c3ae6e09acd16fc50a516975631dd6bb76b8c53)