Pull functest-core when only building childs

Pull functest-core when only building childs

Otherwise it fails if parent and childs containers are not built on
the same host [1].

It would have worked if the docker images were correctly cleant. Then
the last image would have been pulled.

[1] https://build.opnfv.org/ci/job/functest-apex-baremetal-daily-euphrates/362/console

Conflicts:
    build.sh

Change-Id: I293829c839c1cf68c9f7668aa643e16889359fcc
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit e3281b2175ca9983d79ce17de7e7497f749e650e)