Allow building containers from a gerrit change

Allow building containers from a gerrit change

It leverages on git command instead of pip git support [1].
Otherwise it forces pip master which raises side effects in rally [2]

It is mandatory for implementing Functional testing gating [3]

[1] https://pip.pypa.io/en/stable/reference/pip_install/#git
[2] https://gerrit.opnfv.org/gerrit/#/c/40909/
[3] https://wiki.opnfv.org/display/functest/Functional+testing+gating

Conflicts:
    docker/core/Dockerfile

Change-Id: Ia18fee3e60fc08da9b96730ce85d5b7c866144aa
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 84f82d02630c22e8054266c8c0a53ffb805c2071)