github: Add GitHub Actions workflow.

github: Add GitHub Actions workflow.

This is an initial version of GitHub Actions support.  It mostly
mimics our current Travis CI build matrix with slight differences.

Minor difference that we can not install 32-bit version of libunbound
since it is not avaialble in repository.

.travis folder renamed to .ci to highlight that it used not only for
Travis CI.  Travis CI support will be completely removed soon.

What happened to Travis CI:
https://mail.openvswitch.org/pipermail/ovs-dev/2020-November/377773.html

Build with ernel 3.19 dropped as it's not supported and can not be built
with gcc-7+.

Acked-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>